<?xml version="1.0" encoding="UTF-8"?>

<!--XBRL Document Created with the Workiva Platform-->
<!--Copyright 2026 Workiva-->
<!--r:01a159ac-3b62-748d-bf18-ffa919684661,g:37166d96-a963-4bf2-a386-31595d3287cf-->
<link:linkbase xmlns:link="http://www.xbrl.org/2003/linkbase" xmlns:xlink="http://www.w3.org/1999/xlink" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:xbrldt="http://xbrl.org/2005/xbrldt" xsi:schemaLocation="http://www.xbrl.org/2003/linkbase http://www.xbrl.org/2003/xbrl-linkbase-2003-12-31.xsd">
  <link:arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/all" xlink:type="simple" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#all"/>
  <link:arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:type="simple" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#hypercube-dimension"/>
  <link:arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:type="simple" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#dimension-default"/>
  <link:arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:type="simple" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#dimension-domain"/>
  <link:arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/domain-member" xlink:type="simple" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#domain-member"/>
  <link:roleRef roleURI="http://customtruck.com/role/CondensedConsolidatedStatementsofOperationsandComprehensiveIncomeLossunaudited" xlink:type="simple" xlink:href="ctos-20260930.xsd#CondensedConsolidatedStatementsofOperationsandComprehensiveIncomeLossunaudited"/>
  <link:definitionLink xlink:role="http://customtruck.com/role/CondensedConsolidatedStatementsofOperationsandComprehensiveIncomeLossunaudited" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementLineItems_01a159ac-3cde-72ff-b287-5dbec571cff9" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StatementLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementTable_01a159ac-3cde-7fca-b90c-02c4545c3c5a" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StatementTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_StatementLineItems_01a159ac-3cde-72ff-b287-5dbec571cff9" xlink:to="loc_us-gaap_StatementTable_01a159ac-3cde-7fca-b90c-02c4545c3c5a" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ProductOrServiceAxis_01a159ac-3cde-7fb9-8285-e1e6af4824fb" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_ProductOrServiceAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_StatementTable_01a159ac-3cde-7fca-b90c-02c4545c3c5a" xlink:to="loc_srt_ProductOrServiceAxis_01a159ac-3cde-7fb9-8285-e1e6af4824fb" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ProductsAndServicesDomain_01a159ac-3cde-7fb9-8285-e1e6af4824fb_default" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_ProductsAndServicesDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_ProductOrServiceAxis_01a159ac-3cde-7fb9-8285-e1e6af4824fb" xlink:to="loc_srt_ProductsAndServicesDomain_01a159ac-3cde-7fb9-8285-e1e6af4824fb_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ProductsAndServicesDomain_01a159ac-3cde-7e48-a8cf-ad0465ec1387" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_ProductsAndServicesDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_ProductOrServiceAxis_01a159ac-3cde-7fb9-8285-e1e6af4824fb" xlink:to="loc_srt_ProductsAndServicesDomain_01a159ac-3cde-7e48-a8cf-ad0465ec1387" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_ctos_RentalMember_01a159ac-3cde-7c4c-bac5-3100be615276" xlink:href="ctos-20260930.xsd#ctos_RentalMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ProductsAndServicesDomain_01a159ac-3cde-7e48-a8cf-ad0465ec1387" xlink:to="loc_ctos_RentalMember_01a159ac-3cde-7c4c-bac5-3100be615276" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_ctos_EquipmentSalesMember_01a159ac-3cde-781d-b5a9-a60889c328b5" xlink:href="ctos-20260930.xsd#ctos_EquipmentSalesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ProductsAndServicesDomain_01a159ac-3cde-7e48-a8cf-ad0465ec1387" xlink:to="loc_ctos_EquipmentSalesMember_01a159ac-3cde-781d-b5a9-a60889c328b5" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_ctos_PartsSalesAndServicesMember_01a159ac-3cde-7e30-aaf5-ff536fdaaab1" xlink:href="ctos-20260930.xsd#ctos_PartsSalesAndServicesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ProductsAndServicesDomain_01a159ac-3cde-7e48-a8cf-ad0465ec1387" xlink:to="loc_ctos_PartsSalesAndServicesMember_01a159ac-3cde-7e30-aaf5-ff536fdaaab1" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevenuesAbstract_01a159ac-3cde-7bce-97b1-73ff8a3a88f4" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RevenuesAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_01a159ac-3cde-72ff-b287-5dbec571cff9" xlink:to="loc_us-gaap_RevenuesAbstract_01a159ac-3cde-7bce-97b1-73ff8a3a88f4" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_Revenues_01a159ac-3cde-77f4-ae54-47b21ecd6434" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_Revenues"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RevenuesAbstract_01a159ac-3cde-7bce-97b1-73ff8a3a88f4" xlink:to="loc_us-gaap_Revenues_01a159ac-3cde-77f4-ae54-47b21ecd6434" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CostOfGoodsAndServicesSoldAbstract_01a159ac-3cde-7377-994f-11105e9d5188" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CostOfGoodsAndServicesSoldAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_01a159ac-3cde-72ff-b287-5dbec571cff9" xlink:to="loc_us-gaap_CostOfGoodsAndServicesSoldAbstract_01a159ac-3cde-7377-994f-11105e9d5188" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CostOfGoodsAndServiceExcludingDepreciationDepletionAndAmortization_01a159ac-3cde-7195-833f-56578808af2a" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CostOfGoodsAndServiceExcludingDepreciationDepletionAndAmortization"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CostOfGoodsAndServicesSoldAbstract_01a159ac-3cde-7377-994f-11105e9d5188" xlink:to="loc_us-gaap_CostOfGoodsAndServiceExcludingDepreciationDepletionAndAmortization_01a159ac-3cde-7195-833f-56578808af2a" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CostOfGoodsAndServicesSoldDepreciation_01a159ac-3cde-73fc-be62-14e9f5f6643d" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CostOfGoodsAndServicesSoldDepreciation"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CostOfGoodsAndServicesSoldAbstract_01a159ac-3cde-7377-994f-11105e9d5188" xlink:to="loc_us-gaap_CostOfGoodsAndServicesSoldDepreciation_01a159ac-3cde-73fc-be62-14e9f5f6643d" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CostOfRevenue_01a159ac-3cde-7fbc-9251-d1b63ff56d15" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CostOfRevenue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CostOfGoodsAndServicesSoldAbstract_01a159ac-3cde-7377-994f-11105e9d5188" xlink:to="loc_us-gaap_CostOfRevenue_01a159ac-3cde-7fbc-9251-d1b63ff56d15" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_GrossProfit_01a159ac-3cde-7372-b0ae-3c8a021908aa" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_GrossProfit"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_01a159ac-3cde-72ff-b287-5dbec571cff9" xlink:to="loc_us-gaap_GrossProfit_01a159ac-3cde-7372-b0ae-3c8a021908aa" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OperatingExpensesAbstract_01a159ac-3cde-715d-bbe7-b8a1d5c20cbf" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_OperatingExpensesAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_01a159ac-3cde-72ff-b287-5dbec571cff9" xlink:to="loc_us-gaap_OperatingExpensesAbstract_01a159ac-3cde-715d-bbe7-b8a1d5c20cbf" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SellingGeneralAndAdministrativeExpense_01a159ac-3cde-7e4e-97f1-dbb4da8cd957" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SellingGeneralAndAdministrativeExpense"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OperatingExpensesAbstract_01a159ac-3cde-715d-bbe7-b8a1d5c20cbf" xlink:to="loc_us-gaap_SellingGeneralAndAdministrativeExpense_01a159ac-3cde-7e4e-97f1-dbb4da8cd957" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AdjustmentForAmortization_01a159ac-3cde-7435-b1c8-72bc1f36653d" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AdjustmentForAmortization"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OperatingExpensesAbstract_01a159ac-3cde-715d-bbe7-b8a1d5c20cbf" xlink:to="loc_us-gaap_AdjustmentForAmortization_01a159ac-3cde-7435-b1c8-72bc1f36653d" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DepreciationNonproduction_01a159ac-3cde-722f-aa65-7142126768a4" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DepreciationNonproduction"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OperatingExpensesAbstract_01a159ac-3cde-715d-bbe7-b8a1d5c20cbf" xlink:to="loc_us-gaap_DepreciationNonproduction_01a159ac-3cde-722f-aa65-7142126768a4" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_ctos_MergerRelatedCosts_01a159ac-3cde-781e-9f8a-cea0665ef41d" xlink:href="ctos-20260930.xsd#ctos_MergerRelatedCosts"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OperatingExpensesAbstract_01a159ac-3cde-715d-bbe7-b8a1d5c20cbf" xlink:to="loc_ctos_MergerRelatedCosts_01a159ac-3cde-781e-9f8a-cea0665ef41d" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OperatingExpenses_01a159ac-3cde-7052-a0dc-b60f83dc00c3" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_OperatingExpenses"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OperatingExpensesAbstract_01a159ac-3cde-715d-bbe7-b8a1d5c20cbf" xlink:to="loc_us-gaap_OperatingExpenses_01a159ac-3cde-7052-a0dc-b60f83dc00c3" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OperatingIncomeLoss_01a159ac-3cde-79f7-abbf-9f3772f01aad" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_OperatingIncomeLoss"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_01a159ac-3cde-72ff-b287-5dbec571cff9" xlink:to="loc_us-gaap_OperatingIncomeLoss_01a159ac-3cde-79f7-abbf-9f3772f01aad" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NonoperatingIncomeExpenseAbstract_01a159ac-3cde-7005-a0bf-ea78ba90b9ea" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_NonoperatingIncomeExpenseAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_01a159ac-3cde-72ff-b287-5dbec571cff9" xlink:to="loc_us-gaap_NonoperatingIncomeExpenseAbstract_01a159ac-3cde-7005-a0bf-ea78ba90b9ea" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InterestIncomeExpenseNonoperatingNet_01a159ac-3cde-736c-b905-057e5fe06ed3" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_InterestIncomeExpenseNonoperatingNet"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NonoperatingIncomeExpenseAbstract_01a159ac-3cde-7005-a0bf-ea78ba90b9ea" xlink:to="loc_us-gaap_InterestIncomeExpenseNonoperatingNet_01a159ac-3cde-736c-b905-057e5fe06ed3" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherNonoperatingIncomeExpense_01a159ac-3cde-7d6c-b20e-5750cf6e80ef" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_OtherNonoperatingIncomeExpense"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NonoperatingIncomeExpenseAbstract_01a159ac-3cde-7005-a0bf-ea78ba90b9ea" xlink:to="loc_us-gaap_OtherNonoperatingIncomeExpense_01a159ac-3cde-7d6c-b20e-5750cf6e80ef" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NonoperatingIncomeExpense_01a159ac-3cde-739d-a9f3-57b10a7bea9a" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_NonoperatingIncomeExpense"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NonoperatingIncomeExpenseAbstract_01a159ac-3cde-7005-a0bf-ea78ba90b9ea" xlink:to="loc_us-gaap_NonoperatingIncomeExpense_01a159ac-3cde-739d-a9f3-57b10a7bea9a" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeLossFromContinuingOperationsBeforeIncomeTaxesExtraordinaryItemsNoncontrollingInterest_01a159ac-3cde-7cb1-be45-36e7e86461bf" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_IncomeLossFromContinuingOperationsBeforeIncomeTaxesExtraordinaryItemsNoncontrollingInterest"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_01a159ac-3cde-72ff-b287-5dbec571cff9" xlink:to="loc_us-gaap_IncomeLossFromContinuingOperationsBeforeIncomeTaxesExtraordinaryItemsNoncontrollingInterest_01a159ac-3cde-7cb1-be45-36e7e86461bf" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeTaxExpenseBenefit_01a159ac-3cde-79c0-a2ac-28788e4e2391" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_IncomeTaxExpenseBenefit"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_01a159ac-3cde-72ff-b287-5dbec571cff9" xlink:to="loc_us-gaap_IncomeTaxExpenseBenefit_01a159ac-3cde-79c0-a2ac-28788e4e2391" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NetIncomeLoss_01a159ac-3cde-7c49-8475-e1094f531905" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_NetIncomeLoss"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_01a159ac-3cde-72ff-b287-5dbec571cff9" xlink:to="loc_us-gaap_NetIncomeLoss_01a159ac-3cde-7c49-8475-e1094f531905" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherComprehensiveIncomeLossNetOfTaxPeriodIncreaseDecreaseAbstract_01a159ac-3cde-7247-9cde-a62e8943ca1b" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_OtherComprehensiveIncomeLossNetOfTaxPeriodIncreaseDecreaseAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_01a159ac-3cde-72ff-b287-5dbec571cff9" xlink:to="loc_us-gaap_OtherComprehensiveIncomeLossNetOfTaxPeriodIncreaseDecreaseAbstract_01a159ac-3cde-7247-9cde-a62e8943ca1b" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherComprehensiveIncomeForeignCurrencyTransactionAndTranslationAdjustmentNetOfTaxPortionAttributableToParent_01a159ac-3cde-71fa-ba13-5b8733e84993" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_OtherComprehensiveIncomeForeignCurrencyTransactionAndTranslationAdjustmentNetOfTaxPortionAttributableToParent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OtherComprehensiveIncomeLossNetOfTaxPeriodIncreaseDecreaseAbstract_01a159ac-3cde-7247-9cde-a62e8943ca1b" xlink:to="loc_us-gaap_OtherComprehensiveIncomeForeignCurrencyTransactionAndTranslationAdjustmentNetOfTaxPortionAttributableToParent_01a159ac-3cde-71fa-ba13-5b8733e84993" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherComprehensiveIncomeLossNetOfTaxPortionAttributableToParent_01a159ac-3cde-7d3e-a7fa-62e0b33be8c3" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_OtherComprehensiveIncomeLossNetOfTaxPortionAttributableToParent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OtherComprehensiveIncomeLossNetOfTaxPeriodIncreaseDecreaseAbstract_01a159ac-3cde-7247-9cde-a62e8943ca1b" xlink:to="loc_us-gaap_OtherComprehensiveIncomeLossNetOfTaxPortionAttributableToParent_01a159ac-3cde-7d3e-a7fa-62e0b33be8c3" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ComprehensiveIncomeNetOfTax_01a159ac-3cde-7b13-bcda-f9ef23b192d8" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ComprehensiveIncomeNetOfTax"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_01a159ac-3cde-72ff-b287-5dbec571cff9" xlink:to="loc_us-gaap_ComprehensiveIncomeNetOfTax_01a159ac-3cde-7b13-bcda-f9ef23b192d8" xlink:type="arc" order="10"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EarningsPerShareAbstract_01a159ac-3cde-7168-a63e-9d84198c9ca8" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_EarningsPerShareAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_01a159ac-3cde-72ff-b287-5dbec571cff9" xlink:to="loc_us-gaap_EarningsPerShareAbstract_01a159ac-3cde-7168-a63e-9d84198c9ca8" xlink:type="arc" order="11"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EarningsPerShareBasic_01a159ac-3cde-785f-8232-e93630e9abff" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_EarningsPerShareBasic"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EarningsPerShareAbstract_01a159ac-3cde-7168-a63e-9d84198c9ca8" xlink:to="loc_us-gaap_EarningsPerShareBasic_01a159ac-3cde-785f-8232-e93630e9abff" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EarningsPerShareDiluted_01a159ac-3cde-767d-846e-79eade970885" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_EarningsPerShareDiluted"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EarningsPerShareAbstract_01a159ac-3cde-7168-a63e-9d84198c9ca8" xlink:to="loc_us-gaap_EarningsPerShareDiluted_01a159ac-3cde-767d-846e-79eade970885" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_WeightedAverageNumberOfSharesOutstandingDilutedDisclosureItemsAbstract_01a159ac-3cde-72d4-aec0-0c8a9b195504" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_WeightedAverageNumberOfSharesOutstandingDilutedDisclosureItemsAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_01a159ac-3cde-72ff-b287-5dbec571cff9" xlink:to="loc_us-gaap_WeightedAverageNumberOfSharesOutstandingDilutedDisclosureItemsAbstract_01a159ac-3cde-72d4-aec0-0c8a9b195504" xlink:type="arc" order="12"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_WeightedAverageNumberOfSharesOutstandingBasic_01a159ac-3cde-7ad9-9754-8c0c671be0cb" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_WeightedAverageNumberOfSharesOutstandingBasic"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_WeightedAverageNumberOfSharesOutstandingDilutedDisclosureItemsAbstract_01a159ac-3cde-72d4-aec0-0c8a9b195504" xlink:to="loc_us-gaap_WeightedAverageNumberOfSharesOutstandingBasic_01a159ac-3cde-7ad9-9754-8c0c671be0cb" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_WeightedAverageNumberOfDilutedSharesOutstanding_01a159ac-3cde-753f-8995-cd51a33cf268" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_WeightedAverageNumberOfDilutedSharesOutstanding"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_WeightedAverageNumberOfSharesOutstandingDilutedDisclosureItemsAbstract_01a159ac-3cde-72d4-aec0-0c8a9b195504" xlink:to="loc_us-gaap_WeightedAverageNumberOfDilutedSharesOutstanding_01a159ac-3cde-753f-8995-cd51a33cf268" xlink:type="arc" order="2"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://customtruck.com/role/CondensedConsolidatedStatementsofStockholdersEquityunaudited" xlink:type="simple" xlink:href="ctos-20260930.xsd#CondensedConsolidatedStatementsofStockholdersEquityunaudited"/>
  <link:definitionLink xlink:role="http://customtruck.com/role/CondensedConsolidatedStatementsofStockholdersEquityunaudited" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementLineItems_01a159ac-3cde-770c-a788-d846b0815b74" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StatementLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementTable_01a159ac-3cde-7532-998a-021907fe5442" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StatementTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_StatementLineItems_01a159ac-3cde-770c-a788-d846b0815b74" xlink:to="loc_us-gaap_StatementTable_01a159ac-3cde-7532-998a-021907fe5442" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementEquityComponentsAxis_01a159ac-3cde-73b3-8fd3-95eb2fe83cfb" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StatementEquityComponentsAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_StatementTable_01a159ac-3cde-7532-998a-021907fe5442" xlink:to="loc_us-gaap_StatementEquityComponentsAxis_01a159ac-3cde-73b3-8fd3-95eb2fe83cfb" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquityComponentDomain_01a159ac-3cde-73b3-8fd3-95eb2fe83cfb_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_EquityComponentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementEquityComponentsAxis_01a159ac-3cde-73b3-8fd3-95eb2fe83cfb" xlink:to="loc_us-gaap_EquityComponentDomain_01a159ac-3cde-73b3-8fd3-95eb2fe83cfb_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquityComponentDomain_01a159ac-3cde-7472-b7b6-89dfc0ebc74f" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_EquityComponentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementEquityComponentsAxis_01a159ac-3cde-73b3-8fd3-95eb2fe83cfb" xlink:to="loc_us-gaap_EquityComponentDomain_01a159ac-3cde-7472-b7b6-89dfc0ebc74f" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockMember_01a159ac-3cde-7f08-b511-5d3265f25c9b" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CommonStockMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_01a159ac-3cde-7472-b7b6-89dfc0ebc74f" xlink:to="loc_us-gaap_CommonStockMember_01a159ac-3cde-7f08-b511-5d3265f25c9b" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TreasuryStockCommonMember_01a159ac-3cde-79c6-8025-dcf39e4938de" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_TreasuryStockCommonMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_01a159ac-3cde-7472-b7b6-89dfc0ebc74f" xlink:to="loc_us-gaap_TreasuryStockCommonMember_01a159ac-3cde-79c6-8025-dcf39e4938de" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AdditionalPaidInCapitalMember_01a159ac-3cde-7f1c-b9f4-7ec598feb6c9" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AdditionalPaidInCapitalMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_01a159ac-3cde-7472-b7b6-89dfc0ebc74f" xlink:to="loc_us-gaap_AdditionalPaidInCapitalMember_01a159ac-3cde-7f1c-b9f4-7ec598feb6c9" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccumulatedOtherComprehensiveIncomeMember_01a159ac-3cde-7b72-87f4-a58d80c8622f" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AccumulatedOtherComprehensiveIncomeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_01a159ac-3cde-7472-b7b6-89dfc0ebc74f" xlink:to="loc_us-gaap_AccumulatedOtherComprehensiveIncomeMember_01a159ac-3cde-7b72-87f4-a58d80c8622f" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RetainedEarningsMember_01a159ac-3cde-7eb1-8da9-e6390303689c" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RetainedEarningsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_01a159ac-3cde-7472-b7b6-89dfc0ebc74f" xlink:to="loc_us-gaap_RetainedEarningsMember_01a159ac-3cde-7eb1-8da9-e6390303689c" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_01a159ac-3cde-7b79-9190-80100a52d9e1" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_IncreaseDecreaseInStockholdersEquityRollForward"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_01a159ac-3cde-770c-a788-d846b0815b74" xlink:to="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_01a159ac-3cde-7b79-9190-80100a52d9e1" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockSharesIssued_01a159ac-3cde-79c0-959b-e80b4fffbec4" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CommonStockSharesIssued"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_01a159ac-3cde-7b79-9190-80100a52d9e1" xlink:to="loc_us-gaap_CommonStockSharesIssued_01a159ac-3cde-79c0-959b-e80b4fffbec4" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TreasuryStockCommonShares_01a159ac-3cde-7e00-ab9d-1ac8fbd94ae5" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_TreasuryStockCommonShares"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_01a159ac-3cde-7b79-9190-80100a52d9e1" xlink:to="loc_us-gaap_TreasuryStockCommonShares_01a159ac-3cde-7e00-ab9d-1ac8fbd94ae5" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockholdersEquity_01a159ac-3cde-7eb7-a36b-21cdadba0a67" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StockholdersEquity"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_01a159ac-3cde-7b79-9190-80100a52d9e1" xlink:to="loc_us-gaap_StockholdersEquity_01a159ac-3cde-7eb7-a36b-21cdadba0a67" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NetIncomeLoss_01a159ac-3cde-7b8e-bc3b-1940c79beb98" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_NetIncomeLoss"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_01a159ac-3cde-7b79-9190-80100a52d9e1" xlink:to="loc_us-gaap_NetIncomeLoss_01a159ac-3cde-7b8e-bc3b-1940c79beb98" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherComprehensiveIncomeLossNetOfTaxPortionAttributableToParent_01a159ac-3cde-78e4-9e09-a254c257d32c" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_OtherComprehensiveIncomeLossNetOfTaxPortionAttributableToParent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_01a159ac-3cde-7b79-9190-80100a52d9e1" xlink:to="loc_us-gaap_OtherComprehensiveIncomeLossNetOfTaxPortionAttributableToParent_01a159ac-3cde-78e4-9e09-a254c257d32c" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockRepurchasedDuringPeriodShares_01a159ac-3cde-70ea-903f-6695076d387e" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StockRepurchasedDuringPeriodShares"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_01a159ac-3cde-7b79-9190-80100a52d9e1" xlink:to="loc_us-gaap_StockRepurchasedDuringPeriodShares_01a159ac-3cde-70ea-903f-6695076d387e" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockRepurchasedDuringPeriodValue_01a159ac-3cde-710c-8570-723785d7e97c" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StockRepurchasedDuringPeriodValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_01a159ac-3cde-7b79-9190-80100a52d9e1" xlink:to="loc_us-gaap_StockRepurchasedDuringPeriodValue_01a159ac-3cde-710c-8570-723785d7e97c" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_ctos_EarnoutSharesForfeitedDuringPeriodShares_01a15f6d-c7b4-758e-9735-59404ee6cb20" xlink:href="ctos-20260930.xsd#ctos_EarnoutSharesForfeitedDuringPeriodShares"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_01a159ac-3cde-7b79-9190-80100a52d9e1" xlink:to="loc_ctos_EarnoutSharesForfeitedDuringPeriodShares_01a15f6d-c7b4-758e-9735-59404ee6cb20" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockIssuedDuringPeriodSharesShareBasedCompensation_01a159ac-3cde-7200-b629-b28074e31847" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StockIssuedDuringPeriodSharesShareBasedCompensation"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_01a159ac-3cde-7b79-9190-80100a52d9e1" xlink:to="loc_us-gaap_StockIssuedDuringPeriodSharesShareBasedCompensation_01a159ac-3cde-7200-b629-b28074e31847" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockIssuedDuringPeriodValueShareBasedCompensation_01a159ac-3cde-715e-beff-cf3d648abb59" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StockIssuedDuringPeriodValueShareBasedCompensation"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_01a159ac-3cde-7b79-9190-80100a52d9e1" xlink:to="loc_us-gaap_StockIssuedDuringPeriodValueShareBasedCompensation_01a159ac-3cde-715e-beff-cf3d648abb59" xlink:type="arc" order="10"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockSharesIssued_01a159ac-3cde-737e-8985-b3e4d6bbae7a" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CommonStockSharesIssued"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TreasuryStockCommonShares_01a159ac-3cde-73f0-9782-ef9080270922" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_TreasuryStockCommonShares"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockholdersEquity_01a159ac-3cde-7949-8d23-c32ccea216e6" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StockholdersEquity"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://customtruck.com/role/RevenueScheduleofRevenuebyGeographicAreasDetails" xlink:type="simple" xlink:href="ctos-20260930.xsd#RevenueScheduleofRevenuebyGeographicAreasDetails"/>
  <link:definitionLink xlink:role="http://customtruck.com/role/RevenueScheduleofRevenuebyGeographicAreasDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DisaggregationOfRevenueLineItems_01a159ac-3cde-7cb1-9c15-50b2775b4120" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DisaggregationOfRevenueLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DisaggregationOfRevenueTable_01a159ac-3cde-77d7-9e97-5c734443783d" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DisaggregationOfRevenueTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_DisaggregationOfRevenueLineItems_01a159ac-3cde-7cb1-9c15-50b2775b4120" xlink:to="loc_us-gaap_DisaggregationOfRevenueTable_01a159ac-3cde-77d7-9e97-5c734443783d" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_StatementGeographicalAxis_01a159ac-3cde-72db-b813-7d02f9cacb04" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_StatementGeographicalAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DisaggregationOfRevenueTable_01a159ac-3cde-77d7-9e97-5c734443783d" xlink:to="loc_srt_StatementGeographicalAxis_01a159ac-3cde-72db-b813-7d02f9cacb04" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_SegmentGeographicalDomain_01a159ac-3cde-72db-b813-7d02f9cacb04_default" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_SegmentGeographicalDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_StatementGeographicalAxis_01a159ac-3cde-72db-b813-7d02f9cacb04" xlink:to="loc_srt_SegmentGeographicalDomain_01a159ac-3cde-72db-b813-7d02f9cacb04_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_SegmentGeographicalDomain_01a159ac-3cde-738e-a907-318349a7c216" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_SegmentGeographicalDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_StatementGeographicalAxis_01a159ac-3cde-72db-b813-7d02f9cacb04" xlink:to="loc_srt_SegmentGeographicalDomain_01a159ac-3cde-738e-a907-318349a7c216" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_country_US_01a159ac-3cde-78c7-b7b7-e238f10586d3" xlink:href="https://xbrl.sec.gov/country/2026/country-2026.xsd#country_US"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_SegmentGeographicalDomain_01a159ac-3cde-738e-a907-318349a7c216" xlink:to="loc_country_US_01a159ac-3cde-78c7-b7b7-e238f10586d3" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_country_CA_01a159ac-3cde-7d63-a3b6-e66d8a507625" xlink:href="https://xbrl.sec.gov/country/2026/country-2026.xsd#country_CA"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_SegmentGeographicalDomain_01a159ac-3cde-738e-a907-318349a7c216" xlink:to="loc_country_CA_01a159ac-3cde-7d63-a3b6-e66d8a507625" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_Revenues_01a159ac-3cde-7ab7-b799-16aca7faf91d" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_Revenues"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DisaggregationOfRevenueLineItems_01a159ac-3cde-7cb1-9c15-50b2775b4120" xlink:to="loc_us-gaap_Revenues_01a159ac-3cde-7ab7-b799-16aca7faf91d" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://customtruck.com/role/RevenueScheduleofRevenuebyMajorProductandServiceLineDetails" xlink:type="simple" xlink:href="ctos-20260930.xsd#RevenueScheduleofRevenuebyMajorProductandServiceLineDetails"/>
  <link:definitionLink xlink:role="http://customtruck.com/role/RevenueScheduleofRevenuebyMajorProductandServiceLineDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DisaggregationOfRevenueLineItems_01a159ac-3cde-775b-88e4-02b8be544313" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DisaggregationOfRevenueLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DisaggregationOfRevenueTable_01a159ac-3cde-73a9-8461-a6fcbdb2d019" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DisaggregationOfRevenueTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_DisaggregationOfRevenueLineItems_01a159ac-3cde-775b-88e4-02b8be544313" xlink:to="loc_us-gaap_DisaggregationOfRevenueTable_01a159ac-3cde-73a9-8461-a6fcbdb2d019" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ProductOrServiceAxis_01a159ac-3cde-7ace-9cdd-5c51299201f0" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_ProductOrServiceAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DisaggregationOfRevenueTable_01a159ac-3cde-73a9-8461-a6fcbdb2d019" xlink:to="loc_srt_ProductOrServiceAxis_01a159ac-3cde-7ace-9cdd-5c51299201f0" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ProductsAndServicesDomain_01a159ac-3cde-7ace-9cdd-5c51299201f0_default" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_ProductsAndServicesDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_ProductOrServiceAxis_01a159ac-3cde-7ace-9cdd-5c51299201f0" xlink:to="loc_srt_ProductsAndServicesDomain_01a159ac-3cde-7ace-9cdd-5c51299201f0_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ProductsAndServicesDomain_01a159ac-3cde-7d65-8b3c-0710745ba904" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_ProductsAndServicesDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_ProductOrServiceAxis_01a159ac-3cde-7ace-9cdd-5c51299201f0" xlink:to="loc_srt_ProductsAndServicesDomain_01a159ac-3cde-7d65-8b3c-0710745ba904" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_ctos_RentalMember_01a159ac-3cde-79b1-9428-d035905892fa" xlink:href="ctos-20260930.xsd#ctos_RentalMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ProductsAndServicesDomain_01a159ac-3cde-7d65-8b3c-0710745ba904" xlink:to="loc_ctos_RentalMember_01a159ac-3cde-79b1-9428-d035905892fa" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_ctos_RentalRevenueExcludingShippingAndHandlingMember_01a159ac-3cde-7561-9531-271a623d4482" xlink:href="ctos-20260930.xsd#ctos_RentalRevenueExcludingShippingAndHandlingMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_ctos_RentalMember_01a159ac-3cde-79b1-9428-d035905892fa" xlink:to="loc_ctos_RentalRevenueExcludingShippingAndHandlingMember_01a159ac-3cde-7561-9531-271a623d4482" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_ctos_RentalRevenueShippingAndHandlingMember_01a159ac-3cde-7e4a-8723-f921f4b875f9" xlink:href="ctos-20260930.xsd#ctos_RentalRevenueShippingAndHandlingMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_ctos_RentalMember_01a159ac-3cde-79b1-9428-d035905892fa" xlink:to="loc_ctos_RentalRevenueShippingAndHandlingMember_01a159ac-3cde-7e4a-8723-f921f4b875f9" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_ctos_SalesAndServicesMember_01a159ac-3cde-7d80-b236-8057e70fa3e7" xlink:href="ctos-20260930.xsd#ctos_SalesAndServicesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ProductsAndServicesDomain_01a159ac-3cde-7d65-8b3c-0710745ba904" xlink:to="loc_ctos_SalesAndServicesMember_01a159ac-3cde-7d80-b236-8057e70fa3e7" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_ctos_EquipmentSalesMember_01a16a35-b315-7d92-9967-3031c5d4d772" xlink:href="ctos-20260930.xsd#ctos_EquipmentSalesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_ctos_SalesAndServicesMember_01a159ac-3cde-7d80-b236-8057e70fa3e7" xlink:to="loc_ctos_EquipmentSalesMember_01a16a35-b315-7d92-9967-3031c5d4d772" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_ctos_PartsSalesAndServicesMember_01a159ac-3cde-7099-817d-52596e4a52b7" xlink:href="ctos-20260930.xsd#ctos_PartsSalesAndServicesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_ctos_SalesAndServicesMember_01a159ac-3cde-7d80-b236-8057e70fa3e7" xlink:to="loc_ctos_PartsSalesAndServicesMember_01a159ac-3cde-7099-817d-52596e4a52b7" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevenueNotFromContractWithCustomer_01a159ac-3cde-7e0c-8ff4-ebfe0b37b378" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RevenueNotFromContractWithCustomer"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DisaggregationOfRevenueLineItems_01a159ac-3cde-775b-88e4-02b8be544313" xlink:to="loc_us-gaap_RevenueNotFromContractWithCustomer_01a159ac-3cde-7e0c-8ff4-ebfe0b37b378" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax_01a159ac-3cde-75ce-8958-a6fb7f9a9a16" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DisaggregationOfRevenueLineItems_01a159ac-3cde-775b-88e4-02b8be544313" xlink:to="loc_us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax_01a159ac-3cde-75ce-8958-a6fb7f9a9a16" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_Revenues_01a159ac-3cde-78c3-ba06-408e72097a7a" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_Revenues"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DisaggregationOfRevenueLineItems_01a159ac-3cde-775b-88e4-02b8be544313" xlink:to="loc_us-gaap_Revenues_01a159ac-3cde-78c3-ba06-408e72097a7a" xlink:type="arc" order="2"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://customtruck.com/role/InventoryDetails" xlink:type="simple" xlink:href="ctos-20260930.xsd#InventoryDetails"/>
  <link:definitionLink xlink:role="http://customtruck.com/role/InventoryDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InventoryLineItems_01a159ac-3cde-7360-8cc8-40dd78e68465" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_InventoryLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InventoryCurrentTable_01a159ac-3cde-7525-8b66-1d8832662a73" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_InventoryCurrentTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_InventoryLineItems_01a159ac-3cde-7360-8cc8-40dd78e68465" xlink:to="loc_us-gaap_InventoryCurrentTable_01a159ac-3cde-7525-8b66-1d8832662a73" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PublicUtilitiesInventoryAxis_01a159ac-3cde-7f03-aa37-bff99151ebff" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_PublicUtilitiesInventoryAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_InventoryCurrentTable_01a159ac-3cde-7525-8b66-1d8832662a73" xlink:to="loc_us-gaap_PublicUtilitiesInventoryAxis_01a159ac-3cde-7f03-aa37-bff99151ebff" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PublicUtilitiesInventoryTypeDomain_01a159ac-3cde-7f03-aa37-bff99151ebff_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_PublicUtilitiesInventoryTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_PublicUtilitiesInventoryAxis_01a159ac-3cde-7f03-aa37-bff99151ebff" xlink:to="loc_us-gaap_PublicUtilitiesInventoryTypeDomain_01a159ac-3cde-7f03-aa37-bff99151ebff_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PublicUtilitiesInventoryTypeDomain_01a159ac-3cde-7e99-914a-3acb712dfd5b" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_PublicUtilitiesInventoryTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_PublicUtilitiesInventoryAxis_01a159ac-3cde-7f03-aa37-bff99151ebff" xlink:to="loc_us-gaap_PublicUtilitiesInventoryTypeDomain_01a159ac-3cde-7e99-914a-3acb712dfd5b" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_ctos_WholeGoodsMember_01a159ac-3cde-7e2c-a826-7ca778d304b7" xlink:href="ctos-20260930.xsd#ctos_WholeGoodsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PublicUtilitiesInventoryTypeDomain_01a159ac-3cde-7e99-914a-3acb712dfd5b" xlink:to="loc_ctos_WholeGoodsMember_01a159ac-3cde-7e2c-a826-7ca778d304b7" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_ctos_RentalPartsToolsAndAccessoriesMember_01a159ac-3cde-741a-95d6-12f9a85aaac0" xlink:href="ctos-20260930.xsd#ctos_RentalPartsToolsAndAccessoriesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PublicUtilitiesInventoryTypeDomain_01a159ac-3cde-7e99-914a-3acb712dfd5b" xlink:to="loc_ctos_RentalPartsToolsAndAccessoriesMember_01a159ac-3cde-741a-95d6-12f9a85aaac0" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InventoryNet_01a159ac-3cde-7dd4-97ea-9671a8589b97" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_InventoryNet"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_InventoryLineItems_01a159ac-3cde-7360-8cc8-40dd78e68465" xlink:to="loc_us-gaap_InventoryNet_01a159ac-3cde-7dd4-97ea-9671a8589b97" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://customtruck.com/role/FloorPlanFinancingScheduleofFloorPlanPayablesDetails" xlink:type="simple" xlink:href="ctos-20260930.xsd#FloorPlanFinancingScheduleofFloorPlanPayablesDetails"/>
  <link:definitionLink xlink:role="http://customtruck.com/role/FloorPlanFinancingScheduleofFloorPlanPayablesDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentRedemptionLineItems_01a159ac-3cde-7dea-9c65-dda33788fb69" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentRedemptionLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentRedemptionTable_01a159ac-3cde-7c5b-9ef3-56b0357f263a" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentRedemptionTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_DebtInstrumentRedemptionLineItems_01a159ac-3cde-7dea-9c65-dda33788fb69" xlink:to="loc_us-gaap_DebtInstrumentRedemptionTable_01a159ac-3cde-7c5b-9ef3-56b0357f263a" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentAxis_01a159ac-3cde-7e4b-b344-e0a5eb645357" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentRedemptionTable_01a159ac-3cde-7c5b-9ef3-56b0357f263a" xlink:to="loc_us-gaap_DebtInstrumentAxis_01a159ac-3cde-7e4b-b344-e0a5eb645357" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_01a159ac-3cde-7e4b-b344-e0a5eb645357_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_DebtInstrumentAxis_01a159ac-3cde-7e4b-b344-e0a5eb645357" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_01a159ac-3cde-7e4b-b344-e0a5eb645357_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_01a159ac-3cde-7a04-8c33-567d8b352518" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_DebtInstrumentAxis_01a159ac-3cde-7e4b-b344-e0a5eb645357" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_01a159ac-3cde-7a04-8c33-567d8b352518" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_ctos_DaimlerFacilityMember_01a159ac-3cde-7f91-a5e2-6fe8202fc794" xlink:href="ctos-20260930.xsd#ctos_DaimlerFacilityMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_01a159ac-3cde-7a04-8c33-567d8b352518" xlink:to="loc_ctos_DaimlerFacilityMember_01a159ac-3cde-7f91-a5e2-6fe8202fc794" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_ctos_InventoryFinancingAgreementPACCARFinancialCorpMember_01a159ac-3cde-7780-96d9-8a51f824290b" xlink:href="ctos-20260930.xsd#ctos_InventoryFinancingAgreementPACCARFinancialCorpMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_01a159ac-3cde-7a04-8c33-567d8b352518" xlink:to="loc_ctos_InventoryFinancingAgreementPACCARFinancialCorpMember_01a159ac-3cde-7780-96d9-8a51f824290b" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_ctos_FordMotorCreditCompanyFacilityMember_01a159ac-3cde-77e6-95b0-1d9e2401b49a" xlink:href="ctos-20260930.xsd#ctos_FordMotorCreditCompanyFacilityMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_01a159ac-3cde-7a04-8c33-567d8b352518" xlink:to="loc_ctos_FordMotorCreditCompanyFacilityMember_01a159ac-3cde-77e6-95b0-1d9e2401b49a" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_ctos_InventoryLoanGuarantyAndSecurityAgreementMember_01a159ac-3cde-7221-9eac-074e463acc70" xlink:href="ctos-20260930.xsd#ctos_InventoryLoanGuarantyAndSecurityAgreementMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_01a159ac-3cde-7a04-8c33-567d8b352518" xlink:to="loc_ctos_InventoryLoanGuarantyAndSecurityAgreementMember_01a159ac-3cde-7221-9eac-074e463acc70" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_ctos_FloorPlanPayablesTradeCurrent_01a159ac-3cde-7d89-8d90-42426a41f9af" xlink:href="ctos-20260930.xsd#ctos_FloorPlanPayablesTradeCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentRedemptionLineItems_01a159ac-3cde-7dea-9c65-dda33788fb69" xlink:to="loc_ctos_FloorPlanPayablesTradeCurrent_01a159ac-3cde-7d89-8d90-42426a41f9af" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_ctos_FloorPlanPayablesNonTradeCurrent_01a159ac-3cde-7886-a292-36c929e936cf" xlink:href="ctos-20260930.xsd#ctos_FloorPlanPayablesNonTradeCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentRedemptionLineItems_01a159ac-3cde-7dea-9c65-dda33788fb69" xlink:to="loc_ctos_FloorPlanPayablesNonTradeCurrent_01a159ac-3cde-7886-a292-36c929e936cf" xlink:type="arc" order="1"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://customtruck.com/role/FloorPlanFinancingNarrativeDetails" xlink:type="simple" xlink:href="ctos-20260930.xsd#FloorPlanFinancingNarrativeDetails"/>
  <link:definitionLink xlink:role="http://customtruck.com/role/FloorPlanFinancingNarrativeDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentLineItems_01a159ac-3cde-7056-ab52-6d18ed4d3692" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentTable_01a159ac-3cde-70b0-87ef-05881d7e37c6" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_DebtInstrumentLineItems_01a159ac-3cde-7056-ab52-6d18ed4d3692" xlink:to="loc_us-gaap_DebtInstrumentTable_01a159ac-3cde-70b0-87ef-05881d7e37c6" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentAxis_01a159ac-3cde-7a28-a042-2137bf6e5266" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_01a159ac-3cde-70b0-87ef-05881d7e37c6" xlink:to="loc_us-gaap_DebtInstrumentAxis_01a159ac-3cde-7a28-a042-2137bf6e5266" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_01a159ac-3cde-7a28-a042-2137bf6e5266_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_DebtInstrumentAxis_01a159ac-3cde-7a28-a042-2137bf6e5266" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_01a159ac-3cde-7a28-a042-2137bf6e5266_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_01a159ac-3cde-726e-8a20-8d89bc9e0058" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_DebtInstrumentAxis_01a159ac-3cde-7a28-a042-2137bf6e5266" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_01a159ac-3cde-726e-8a20-8d89bc9e0058" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_ctos_FloorPlanFinancingArrangementsMember_01a159ac-3cde-7c27-8abc-535e43edd965" xlink:href="ctos-20260930.xsd#ctos_FloorPlanFinancingArrangementsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_01a159ac-3cde-726e-8a20-8d89bc9e0058" xlink:to="loc_ctos_FloorPlanFinancingArrangementsMember_01a159ac-3cde-7c27-8abc-535e43edd965" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_ctos_DaimlerFacilityMember_01a159ac-3cde-7634-92e8-c7a22a0be3fa" xlink:href="ctos-20260930.xsd#ctos_DaimlerFacilityMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_ctos_FloorPlanFinancingArrangementsMember_01a159ac-3cde-7c27-8abc-535e43edd965" xlink:to="loc_ctos_DaimlerFacilityMember_01a159ac-3cde-7634-92e8-c7a22a0be3fa" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_ctos_InventoryFinancingAgreementPACCARFinancialCorpMember_01a159ac-3cde-7400-aa3e-f10b44f05922" xlink:href="ctos-20260930.xsd#ctos_InventoryFinancingAgreementPACCARFinancialCorpMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_ctos_FloorPlanFinancingArrangementsMember_01a159ac-3cde-7c27-8abc-535e43edd965" xlink:to="loc_ctos_InventoryFinancingAgreementPACCARFinancialCorpMember_01a159ac-3cde-7400-aa3e-f10b44f05922" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_ctos_FordMotorCreditCompanyFacilityMember_01a159ac-3cde-7923-9f35-6a1f8fc96265" xlink:href="ctos-20260930.xsd#ctos_FordMotorCreditCompanyFacilityMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_ctos_FloorPlanFinancingArrangementsMember_01a159ac-3cde-7c27-8abc-535e43edd965" xlink:to="loc_ctos_FordMotorCreditCompanyFacilityMember_01a159ac-3cde-7923-9f35-6a1f8fc96265" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_ctos_InventoryLoanGuarantyAndSecurityAgreementMember_01a159ac-3cde-783e-b3e6-64606e05b270" xlink:href="ctos-20260930.xsd#ctos_InventoryLoanGuarantyAndSecurityAgreementMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_ctos_FloorPlanFinancingArrangementsMember_01a159ac-3cde-7c27-8abc-535e43edd965" xlink:to="loc_ctos_InventoryLoanGuarantyAndSecurityAgreementMember_01a159ac-3cde-783e-b3e6-64606e05b270" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeAxis_01a159ac-3cde-70f5-aa66-c05d5e78567f" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LongtermDebtTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_01a159ac-3cde-70b0-87ef-05881d7e37c6" xlink:to="loc_us-gaap_LongtermDebtTypeAxis_01a159ac-3cde-70f5-aa66-c05d5e78567f" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeDomain_01a159ac-3cde-70f5-aa66-c05d5e78567f_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LongtermDebtTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_LongtermDebtTypeAxis_01a159ac-3cde-70f5-aa66-c05d5e78567f" xlink:to="loc_us-gaap_LongtermDebtTypeDomain_01a159ac-3cde-70f5-aa66-c05d5e78567f_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeDomain_01a159ac-3cde-7b93-8bbe-0cbb5c78b0af" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LongtermDebtTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_LongtermDebtTypeAxis_01a159ac-3cde-70f5-aa66-c05d5e78567f" xlink:to="loc_us-gaap_LongtermDebtTypeDomain_01a159ac-3cde-7b93-8bbe-0cbb5c78b0af" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LineOfCreditMember_01a159ac-3cde-73a1-b231-68f2cd96c833" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LineOfCreditMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LongtermDebtTypeDomain_01a159ac-3cde-7b93-8bbe-0cbb5c78b0af" xlink:to="loc_us-gaap_LineOfCreditMember_01a159ac-3cde-73a1-b231-68f2cd96c833" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_VariableRateAxis_01a159ac-3cde-7bf3-a4e4-f60ca02bacb1" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_VariableRateAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_01a159ac-3cde-70b0-87ef-05881d7e37c6" xlink:to="loc_us-gaap_VariableRateAxis_01a159ac-3cde-7bf3-a4e4-f60ca02bacb1" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_VariableRateDomain_01a159ac-3cde-7bf3-a4e4-f60ca02bacb1_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_VariableRateDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_VariableRateAxis_01a159ac-3cde-7bf3-a4e4-f60ca02bacb1" xlink:to="loc_us-gaap_VariableRateDomain_01a159ac-3cde-7bf3-a4e4-f60ca02bacb1_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_VariableRateDomain_01a159ac-3cde-758b-9cbc-858f3540a896" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_VariableRateDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_VariableRateAxis_01a159ac-3cde-7bf3-a4e4-f60ca02bacb1" xlink:to="loc_us-gaap_VariableRateDomain_01a159ac-3cde-758b-9cbc-858f3540a896" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PrimeRateMember_01a159ac-3cde-7f24-9b9b-d85de19fee39" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_PrimeRateMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_VariableRateDomain_01a159ac-3cde-758b-9cbc-858f3540a896" xlink:to="loc_us-gaap_PrimeRateMember_01a159ac-3cde-7f24-9b9b-d85de19fee39" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SecuredOvernightFinancingRateSofrMember_01a159ac-3cde-77a8-be83-c1a4c0caf8b3" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SecuredOvernightFinancingRateSofrMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_VariableRateDomain_01a159ac-3cde-758b-9cbc-858f3540a896" xlink:to="loc_us-gaap_SecuredOvernightFinancingRateSofrMember_01a159ac-3cde-77a8-be83-c1a4c0caf8b3" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CreditFacilityAxis_01a159ac-3cde-776e-a571-255c465be615" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CreditFacilityAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_01a159ac-3cde-70b0-87ef-05881d7e37c6" xlink:to="loc_us-gaap_CreditFacilityAxis_01a159ac-3cde-776e-a571-255c465be615" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CreditFacilityDomain_01a159ac-3cde-776e-a571-255c465be615_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CreditFacilityDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_CreditFacilityAxis_01a159ac-3cde-776e-a571-255c465be615" xlink:to="loc_us-gaap_CreditFacilityDomain_01a159ac-3cde-776e-a571-255c465be615_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CreditFacilityDomain_01a159ac-3cde-71b7-8c04-a0cc97bf6666" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CreditFacilityDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_CreditFacilityAxis_01a159ac-3cde-776e-a571-255c465be615" xlink:to="loc_us-gaap_CreditFacilityDomain_01a159ac-3cde-71b7-8c04-a0cc97bf6666" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevolvingCreditFacilityMember_01a159ac-3cde-7892-bb80-20dac8c24615" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RevolvingCreditFacilityMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CreditFacilityDomain_01a159ac-3cde-71b7-8c04-a0cc97bf6666" xlink:to="loc_us-gaap_RevolvingCreditFacilityMember_01a159ac-3cde-7892-bb80-20dac8c24615" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LineOfCreditFacilityAxis_01a159ac-3cde-7993-9cb7-63c05b571e91" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LineOfCreditFacilityAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_01a159ac-3cde-70b0-87ef-05881d7e37c6" xlink:to="loc_us-gaap_LineOfCreditFacilityAxis_01a159ac-3cde-7993-9cb7-63c05b571e91" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LineOfCreditFacilityLenderDomain_01a159ac-3cde-7993-9cb7-63c05b571e91_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LineOfCreditFacilityLenderDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_LineOfCreditFacilityAxis_01a159ac-3cde-7993-9cb7-63c05b571e91" xlink:to="loc_us-gaap_LineOfCreditFacilityLenderDomain_01a159ac-3cde-7993-9cb7-63c05b571e91_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LineOfCreditFacilityLenderDomain_01a159ac-3cde-7dd6-80c8-aa1300b13376" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LineOfCreditFacilityLenderDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_LineOfCreditFacilityAxis_01a159ac-3cde-7993-9cb7-63c05b571e91" xlink:to="loc_us-gaap_LineOfCreditFacilityLenderDomain_01a159ac-3cde-7dd6-80c8-aa1300b13376" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_ctos_PNCEquipmentFinanceLLCMember_01a159ac-3cde-75a1-ada2-87cadfc879ef" xlink:href="ctos-20260930.xsd#ctos_PNCEquipmentFinanceLLCMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LineOfCreditFacilityLenderDomain_01a159ac-3cde-7dd6-80c8-aa1300b13376" xlink:to="loc_ctos_PNCEquipmentFinanceLLCMember_01a159ac-3cde-75a1-ada2-87cadfc879ef" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InterestExpenseDebt_01a159ac-3cde-7f17-8da2-93695a414414" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_InterestExpenseDebt"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_01a159ac-3cde-7056-ab52-6d18ed4d3692" xlink:to="loc_us-gaap_InterestExpenseDebt_01a159ac-3cde-7f17-8da2-93695a414414" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_ctos_DebtInstrumentInterestFreePeriod_01a159ac-3cde-788f-abd9-2208ba6c2a62" xlink:href="ctos-20260930.xsd#ctos_DebtInstrumentInterestFreePeriod"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_01a159ac-3cde-7056-ab52-6d18ed4d3692" xlink:to="loc_ctos_DebtInstrumentInterestFreePeriod_01a159ac-3cde-788f-abd9-2208ba6c2a62" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity_01a159ac-3cde-759d-80c0-eebab4299e98" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_01a159ac-3cde-7056-ab52-6d18ed4d3692" xlink:to="loc_us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity_01a159ac-3cde-759d-80c0-eebab4299e98" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentBasisSpreadOnVariableRate1_01a159ac-3cde-7cb9-a512-99b5c74fee0f" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentBasisSpreadOnVariableRate1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_01a159ac-3cde-7056-ab52-6d18ed4d3692" xlink:to="loc_us-gaap_DebtInstrumentBasisSpreadOnVariableRate1_01a159ac-3cde-7cb9-a512-99b5c74fee0f" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentTerm_01a16a56-45bf-7019-9d5f-c816cef73597" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentTerm"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_01a159ac-3cde-7056-ab52-6d18ed4d3692" xlink:to="loc_us-gaap_DebtInstrumentTerm_01a16a56-45bf-7019-9d5f-c816cef73597" xlink:type="arc" order="4"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://customtruck.com/role/LongTermDebtScheduleofDebtObligationsandAssociatedInterestRatesDetails" xlink:type="simple" xlink:href="ctos-20260930.xsd#LongTermDebtScheduleofDebtObligationsandAssociatedInterestRatesDetails"/>
  <link:definitionLink xlink:role="http://customtruck.com/role/LongTermDebtScheduleofDebtObligationsandAssociatedInterestRatesDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentLineItems_01a159ac-3da2-7f32-a2b6-afd7c7f41bfe" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentTable_01a159ac-3da2-75e5-b443-9b8c53c7f45e" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_DebtInstrumentLineItems_01a159ac-3da2-7f32-a2b6-afd7c7f41bfe" xlink:to="loc_us-gaap_DebtInstrumentTable_01a159ac-3da2-75e5-b443-9b8c53c7f45e" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentAxis_01a159ac-3da2-7564-b3a8-d77033cfc2d7" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_01a159ac-3da2-75e5-b443-9b8c53c7f45e" xlink:to="loc_us-gaap_DebtInstrumentAxis_01a159ac-3da2-7564-b3a8-d77033cfc2d7" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_01a159ac-3da2-7564-b3a8-d77033cfc2d7_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_DebtInstrumentAxis_01a159ac-3da2-7564-b3a8-d77033cfc2d7" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_01a159ac-3da2-7564-b3a8-d77033cfc2d7_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_01a159ac-3da2-7c86-aec0-bc101ae5ac7c" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_DebtInstrumentAxis_01a159ac-3da2-7564-b3a8-d77033cfc2d7" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_01a159ac-3da2-7c86-aec0-bc101ae5ac7c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_ctos_ABLFacilityMember_01a159ac-3da2-7383-b1bb-a7e5b2e73b07" xlink:href="ctos-20260930.xsd#ctos_ABLFacilityMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_01a159ac-3da2-7c86-aec0-bc101ae5ac7c" xlink:to="loc_ctos_ABLFacilityMember_01a159ac-3da2-7383-b1bb-a7e5b2e73b07" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_ctos_A550SeniorSecuredSecondLienNotesDue2029Member_01a159ac-3da2-702c-8c3c-bcb0c362e672" xlink:href="ctos-20260930.xsd#ctos_A550SeniorSecuredSecondLienNotesDue2029Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_01a159ac-3da2-7c86-aec0-bc101ae5ac7c" xlink:to="loc_ctos_A550SeniorSecuredSecondLienNotesDue2029Member_01a159ac-3da2-702c-8c3c-bcb0c362e672" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_ctos_A2023CreditFacilityMember_01a159ac-3da2-7cec-99a0-28b7fc256b0e" xlink:href="ctos-20260930.xsd#ctos_A2023CreditFacilityMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_01a159ac-3da2-7c86-aec0-bc101ae5ac7c" xlink:to="loc_ctos_A2023CreditFacilityMember_01a159ac-3da2-7cec-99a0-28b7fc256b0e" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CreditFacilityAxis_01a159ac-3da2-7a85-8395-377500ec20b1" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CreditFacilityAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_01a159ac-3da2-75e5-b443-9b8c53c7f45e" xlink:to="loc_us-gaap_CreditFacilityAxis_01a159ac-3da2-7a85-8395-377500ec20b1" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CreditFacilityDomain_01a159ac-3da2-7a85-8395-377500ec20b1_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CreditFacilityDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_CreditFacilityAxis_01a159ac-3da2-7a85-8395-377500ec20b1" xlink:to="loc_us-gaap_CreditFacilityDomain_01a159ac-3da2-7a85-8395-377500ec20b1_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CreditFacilityDomain_01a159ac-3da2-7232-9ea8-a9e050141db1" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CreditFacilityDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_CreditFacilityAxis_01a159ac-3da2-7a85-8395-377500ec20b1" xlink:to="loc_us-gaap_CreditFacilityDomain_01a159ac-3da2-7232-9ea8-a9e050141db1" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevolvingCreditFacilityMember_01a159ac-3da2-73e2-b48f-3e99f6693df6" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RevolvingCreditFacilityMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CreditFacilityDomain_01a159ac-3da2-7232-9ea8-a9e050141db1" xlink:to="loc_us-gaap_RevolvingCreditFacilityMember_01a159ac-3da2-73e2-b48f-3e99f6693df6" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeAxis_01a159ac-3da2-7de6-b39a-81f480a41242" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LongtermDebtTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_01a159ac-3da2-75e5-b443-9b8c53c7f45e" xlink:to="loc_us-gaap_LongtermDebtTypeAxis_01a159ac-3da2-7de6-b39a-81f480a41242" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeDomain_01a159ac-3da2-7de6-b39a-81f480a41242_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LongtermDebtTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_LongtermDebtTypeAxis_01a159ac-3da2-7de6-b39a-81f480a41242" xlink:to="loc_us-gaap_LongtermDebtTypeDomain_01a159ac-3da2-7de6-b39a-81f480a41242_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeDomain_01a159ac-3da2-744c-a58d-89dae1ee0ef3" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LongtermDebtTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_LongtermDebtTypeAxis_01a159ac-3da2-7de6-b39a-81f480a41242" xlink:to="loc_us-gaap_LongtermDebtTypeDomain_01a159ac-3da2-744c-a58d-89dae1ee0ef3" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LineOfCreditMember_01a159ac-3da2-78d8-b8bc-373a3d96684f" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LineOfCreditMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LongtermDebtTypeDomain_01a159ac-3da2-744c-a58d-89dae1ee0ef3" xlink:to="loc_us-gaap_LineOfCreditMember_01a159ac-3da2-78d8-b8bc-373a3d96684f" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SeniorNotesMember_01a159ac-3da2-7d31-8eae-57b1aa761571" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SeniorNotesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LongtermDebtTypeDomain_01a159ac-3da2-744c-a58d-89dae1ee0ef3" xlink:to="loc_us-gaap_SeniorNotesMember_01a159ac-3da2-7d31-8eae-57b1aa761571" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NotesPayableOtherPayablesMember_01a159ac-3da2-7893-999e-e9696056a38b" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_NotesPayableOtherPayablesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LongtermDebtTypeDomain_01a159ac-3da2-744c-a58d-89dae1ee0ef3" xlink:to="loc_us-gaap_NotesPayableOtherPayablesMember_01a159ac-3da2-7893-999e-e9696056a38b" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeAxis_01a159ac-3da2-7abb-98d6-243b5032d707" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_RangeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_01a159ac-3da2-75e5-b443-9b8c53c7f45e" xlink:to="loc_srt_RangeAxis_01a159ac-3da2-7abb-98d6-243b5032d707" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_01a159ac-3da2-7abb-98d6-243b5032d707_default" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_RangeAxis_01a159ac-3da2-7abb-98d6-243b5032d707" xlink:to="loc_srt_RangeMember_01a159ac-3da2-7abb-98d6-243b5032d707_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_01a159ac-3da2-7a12-87f0-c2d39d28a429" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_RangeAxis_01a159ac-3da2-7abb-98d6-243b5032d707" xlink:to="loc_srt_RangeMember_01a159ac-3da2-7a12-87f0-c2d39d28a429" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MinimumMember_01a159ac-3da2-7da1-980a-7cee460f8f88" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_MinimumMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_01a159ac-3da2-7a12-87f0-c2d39d28a429" xlink:to="loc_srt_MinimumMember_01a159ac-3da2-7da1-980a-7cee460f8f88" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MaximumMember_01a159ac-3da2-7ec9-948a-8fc5da7ab479" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_MaximumMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_01a159ac-3da2-7a12-87f0-c2d39d28a429" xlink:to="loc_srt_MaximumMember_01a159ac-3da2-7ec9-948a-8fc5da7ab479" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentCarryingAmount_01a159ac-3da2-7968-b719-a9897430ec26" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentCarryingAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_01a159ac-3da2-7f32-a2b6-afd7c7f41bfe" xlink:to="loc_us-gaap_DebtInstrumentCarryingAmount_01a159ac-3da2-7968-b719-a9897430ec26" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentInterestRateStatedPercentage_01a159ac-3da2-7dc4-802c-1c5c15c4ce26" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentInterestRateStatedPercentage"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_01a159ac-3da2-7f32-a2b6-afd7c7f41bfe" xlink:to="loc_us-gaap_DebtInstrumentInterestRateStatedPercentage_01a159ac-3da2-7dc4-802c-1c5c15c4ce26" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentUnamortizedDiscountPremiumAndDebtIssuanceCostsNet_01a159ac-3da2-797f-b308-9d19f006ca37" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentUnamortizedDiscountPremiumAndDebtIssuanceCostsNet"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_01a159ac-3da2-7f32-a2b6-afd7c7f41bfe" xlink:to="loc_us-gaap_DebtInstrumentUnamortizedDiscountPremiumAndDebtIssuanceCostsNet_01a159ac-3da2-797f-b308-9d19f006ca37" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongTermDebt_01a159ac-3da2-7ecf-82eb-de3666abbf99" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LongTermDebt"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_01a159ac-3da2-7f32-a2b6-afd7c7f41bfe" xlink:to="loc_us-gaap_LongTermDebt_01a159ac-3da2-7ecf-82eb-de3666abbf99" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongTermDebtCurrent_01a159ac-3da2-7daa-9cf8-f06456a2e493" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LongTermDebtCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_01a159ac-3da2-7f32-a2b6-afd7c7f41bfe" xlink:to="loc_us-gaap_LongTermDebtCurrent_01a159ac-3da2-7daa-9cf8-f06456a2e493" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongTermDebtNoncurrent_01a159ac-3da2-7c4a-951f-6b8b6082dd85" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LongTermDebtNoncurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_01a159ac-3da2-7f32-a2b6-afd7c7f41bfe" xlink:to="loc_us-gaap_LongTermDebtNoncurrent_01a159ac-3da2-7c4a-951f-6b8b6082dd85" xlink:type="arc" order="5"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://customtruck.com/role/LongTermDebtNarrativeDetails" xlink:type="simple" xlink:href="ctos-20260930.xsd#LongTermDebtNarrativeDetails"/>
  <link:definitionLink xlink:role="http://customtruck.com/role/LongTermDebtNarrativeDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentLineItems_01a159ac-3da2-74ac-92d3-2a8d2556e14b" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentTable_01a159ac-3da2-7bc8-8881-82b57e5f8c53" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_DebtInstrumentLineItems_01a159ac-3da2-74ac-92d3-2a8d2556e14b" xlink:to="loc_us-gaap_DebtInstrumentTable_01a159ac-3da2-7bc8-8881-82b57e5f8c53" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CreditFacilityAxis_01a159ac-3da2-7d93-92dd-bb89bac0056a" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CreditFacilityAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_01a159ac-3da2-7bc8-8881-82b57e5f8c53" xlink:to="loc_us-gaap_CreditFacilityAxis_01a159ac-3da2-7d93-92dd-bb89bac0056a" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CreditFacilityDomain_01a159ac-3da2-7d93-92dd-bb89bac0056a_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CreditFacilityDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_CreditFacilityAxis_01a159ac-3da2-7d93-92dd-bb89bac0056a" xlink:to="loc_us-gaap_CreditFacilityDomain_01a159ac-3da2-7d93-92dd-bb89bac0056a_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CreditFacilityDomain_01a159ac-3da2-77f1-bdba-b5a3bc9a95d9" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CreditFacilityDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_CreditFacilityAxis_01a159ac-3da2-7d93-92dd-bb89bac0056a" xlink:to="loc_us-gaap_CreditFacilityDomain_01a159ac-3da2-77f1-bdba-b5a3bc9a95d9" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevolvingCreditFacilityMember_01a159ac-3da2-7c8e-a992-21ad7b137b6b" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RevolvingCreditFacilityMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CreditFacilityDomain_01a159ac-3da2-77f1-bdba-b5a3bc9a95d9" xlink:to="loc_us-gaap_RevolvingCreditFacilityMember_01a159ac-3da2-7c8e-a992-21ad7b137b6b" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LetterOfCreditMember_01a159ac-3da2-7f9c-a975-552fe87bf86f" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LetterOfCreditMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CreditFacilityDomain_01a159ac-3da2-77f1-bdba-b5a3bc9a95d9" xlink:to="loc_us-gaap_LetterOfCreditMember_01a159ac-3da2-7f9c-a975-552fe87bf86f" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentAxis_01a159ac-3da2-7765-8ce0-fb55a33c8d78" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_01a159ac-3da2-7bc8-8881-82b57e5f8c53" xlink:to="loc_us-gaap_DebtInstrumentAxis_01a159ac-3da2-7765-8ce0-fb55a33c8d78" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_01a159ac-3da2-7765-8ce0-fb55a33c8d78_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_DebtInstrumentAxis_01a159ac-3da2-7765-8ce0-fb55a33c8d78" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_01a159ac-3da2-7765-8ce0-fb55a33c8d78_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_01a159ac-3da2-790a-9410-238f76d6ffce" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_DebtInstrumentAxis_01a159ac-3da2-7765-8ce0-fb55a33c8d78" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_01a159ac-3da2-790a-9410-238f76d6ffce" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_ctos_ABLFacilityMember_01a159ac-3da2-7b69-ba15-f32b335125a9" xlink:href="ctos-20260930.xsd#ctos_ABLFacilityMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_01a159ac-3da2-790a-9410-238f76d6ffce" xlink:to="loc_ctos_ABLFacilityMember_01a159ac-3da2-7b69-ba15-f32b335125a9" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_ctos_SBKCLoanAgreementMember_01a159ac-3da2-706a-a0a1-9c7f76c5a6ae" xlink:href="ctos-20260930.xsd#ctos_SBKCLoanAgreementMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_01a159ac-3da2-790a-9410-238f76d6ffce" xlink:to="loc_ctos_SBKCLoanAgreementMember_01a159ac-3da2-706a-a0a1-9c7f76c5a6ae" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeAxis_01a159ac-3da2-7471-b1c0-276865977eae" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_RangeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_01a159ac-3da2-7bc8-8881-82b57e5f8c53" xlink:to="loc_srt_RangeAxis_01a159ac-3da2-7471-b1c0-276865977eae" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_01a159ac-3da2-7471-b1c0-276865977eae_default" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_RangeAxis_01a159ac-3da2-7471-b1c0-276865977eae" xlink:to="loc_srt_RangeMember_01a159ac-3da2-7471-b1c0-276865977eae_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_01a159ac-3da2-772a-bcef-93ac01ca2430" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_RangeAxis_01a159ac-3da2-7471-b1c0-276865977eae" xlink:to="loc_srt_RangeMember_01a159ac-3da2-772a-bcef-93ac01ca2430" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MinimumMember_01a159ac-3da2-7877-96bf-8d2ba6726224" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_MinimumMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_01a159ac-3da2-772a-bcef-93ac01ca2430" xlink:to="loc_srt_MinimumMember_01a159ac-3da2-7877-96bf-8d2ba6726224" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MaximumMember_01a159ac-3da2-7877-b3c4-17b9222f4138" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_MaximumMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_01a159ac-3da2-772a-bcef-93ac01ca2430" xlink:to="loc_srt_MaximumMember_01a159ac-3da2-7877-b3c4-17b9222f4138" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_VariableRateAxis_01a159ac-3da2-79db-8305-644dc8ecac1e" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_VariableRateAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_01a159ac-3da2-7bc8-8881-82b57e5f8c53" xlink:to="loc_us-gaap_VariableRateAxis_01a159ac-3da2-79db-8305-644dc8ecac1e" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_VariableRateDomain_01a159ac-3da2-79db-8305-644dc8ecac1e_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_VariableRateDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_VariableRateAxis_01a159ac-3da2-79db-8305-644dc8ecac1e" xlink:to="loc_us-gaap_VariableRateDomain_01a159ac-3da2-79db-8305-644dc8ecac1e_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_VariableRateDomain_01a159ac-3da2-7b91-a7e5-34b5f64b2d14" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_VariableRateDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_VariableRateAxis_01a159ac-3da2-79db-8305-644dc8ecac1e" xlink:to="loc_us-gaap_VariableRateDomain_01a159ac-3da2-7b91-a7e5-34b5f64b2d14" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BaseRateMember_01a159ac-3da2-7f58-b448-f71fdaeb7507" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_BaseRateMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_VariableRateDomain_01a159ac-3da2-7b91-a7e5-34b5f64b2d14" xlink:to="loc_us-gaap_BaseRateMember_01a159ac-3da2-7f58-b448-f71fdaeb7507" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SecuredOvernightFinancingRateSofrMember_01a159ac-3da2-7bba-b123-7652309d0a4c" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SecuredOvernightFinancingRateSofrMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_VariableRateDomain_01a159ac-3da2-7b91-a7e5-34b5f64b2d14" xlink:to="loc_us-gaap_SecuredOvernightFinancingRateSofrMember_01a159ac-3da2-7bba-b123-7652309d0a4c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_ctos_TermCanadianOvernightRepoRateAverageCORRAMember_01a159ac-3da2-7bec-919d-67c4f84404f4" xlink:href="ctos-20260930.xsd#ctos_TermCanadianOvernightRepoRateAverageCORRAMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_VariableRateDomain_01a159ac-3da2-7b91-a7e5-34b5f64b2d14" xlink:to="loc_ctos_TermCanadianOvernightRepoRateAverageCORRAMember_01a159ac-3da2-7bec-919d-67c4f84404f4" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeAxis_01a159ac-3da2-7c14-a8d8-870dbf12091f" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LongtermDebtTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_01a159ac-3da2-7bc8-8881-82b57e5f8c53" xlink:to="loc_us-gaap_LongtermDebtTypeAxis_01a159ac-3da2-7c14-a8d8-870dbf12091f" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeDomain_01a159ac-3da2-7c14-a8d8-870dbf12091f_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LongtermDebtTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_LongtermDebtTypeAxis_01a159ac-3da2-7c14-a8d8-870dbf12091f" xlink:to="loc_us-gaap_LongtermDebtTypeDomain_01a159ac-3da2-7c14-a8d8-870dbf12091f_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeDomain_01a159ac-3da2-797c-89cf-449f634daee0" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LongtermDebtTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_LongtermDebtTypeAxis_01a159ac-3da2-7c14-a8d8-870dbf12091f" xlink:to="loc_us-gaap_LongtermDebtTypeDomain_01a159ac-3da2-797c-89cf-449f634daee0" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NotesPayableOtherPayablesMember_01a159ac-3da2-7d10-8771-bc73d515bf8e" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_NotesPayableOtherPayablesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LongtermDebtTypeDomain_01a159ac-3da2-797c-89cf-449f634daee0" xlink:to="loc_us-gaap_NotesPayableOtherPayablesMember_01a159ac-3da2-7d10-8771-bc73d515bf8e" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LineOfCreditFacilityRemainingBorrowingCapacity_01a159ac-3da2-775e-baf3-98e13c984dc8" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LineOfCreditFacilityRemainingBorrowingCapacity"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_01a159ac-3da2-74ac-92d3-2a8d2556e14b" xlink:to="loc_us-gaap_LineOfCreditFacilityRemainingBorrowingCapacity_01a159ac-3da2-775e-baf3-98e13c984dc8" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LineOfCredit_01a159ac-3da2-78ca-a305-439c092ff16b" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LineOfCredit"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_01a159ac-3da2-74ac-92d3-2a8d2556e14b" xlink:to="loc_us-gaap_LineOfCredit_01a159ac-3da2-78ca-a305-439c092ff16b" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity_01a159ac-3da2-75ae-af13-af08170ddf09" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_01a159ac-3da2-74ac-92d3-2a8d2556e14b" xlink:to="loc_us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity_01a159ac-3da2-75ae-af13-af08170ddf09" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentBasisSpreadOnVariableRate1_01a159ac-3da2-7d51-81c0-f3118f01d487" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentBasisSpreadOnVariableRate1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_01a159ac-3da2-74ac-92d3-2a8d2556e14b" xlink:to="loc_us-gaap_DebtInstrumentBasisSpreadOnVariableRate1_01a159ac-3da2-7d51-81c0-f3118f01d487" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentCarryingAmount_01a159ac-3da2-7378-aab8-a7821692f626" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentCarryingAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_01a159ac-3da2-74ac-92d3-2a8d2556e14b" xlink:to="loc_us-gaap_DebtInstrumentCarryingAmount_01a159ac-3da2-7378-aab8-a7821692f626" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_ctos_DebtInstrumentRenewalTerm_01a159ac-3da2-7bce-93bc-783ed0c1d512" xlink:href="ctos-20260930.xsd#ctos_DebtInstrumentRenewalTerm"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_01a159ac-3da2-74ac-92d3-2a8d2556e14b" xlink:to="loc_ctos_DebtInstrumentRenewalTerm_01a159ac-3da2-7bce-93bc-783ed0c1d512" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentInterestRateStatedPercentage_01a159ac-3da2-72f2-ab40-6023e7f9a1fa" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentInterestRateStatedPercentage"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_01a159ac-3da2-74ac-92d3-2a8d2556e14b" xlink:to="loc_us-gaap_DebtInstrumentInterestRateStatedPercentage_01a159ac-3da2-72f2-ab40-6023e7f9a1fa" xlink:type="arc" order="6"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://customtruck.com/role/EquityDetails" xlink:type="simple" xlink:href="ctos-20260930.xsd#EquityDetails"/>
  <link:definitionLink xlink:role="http://customtruck.com/role/EquityDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_ctos_EquityLineItems_01a159ac-3da2-7340-b903-cce9b994688c" xlink:href="ctos-20260930.xsd#ctos_EquityLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_ctos_ScheduleOfEquityTable_01a159ac-3da2-7552-b051-e71f680ab29c" xlink:href="ctos-20260930.xsd#ctos_ScheduleOfEquityTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_ctos_EquityLineItems_01a159ac-3da2-7340-b903-cce9b994688c" xlink:to="loc_ctos_ScheduleOfEquityTable_01a159ac-3da2-7552-b051-e71f680ab29c" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementEquityComponentsAxis_01a159ac-3da2-7332-98e0-e39b4ecaf947" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StatementEquityComponentsAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_ctos_ScheduleOfEquityTable_01a159ac-3da2-7552-b051-e71f680ab29c" xlink:to="loc_us-gaap_StatementEquityComponentsAxis_01a159ac-3da2-7332-98e0-e39b4ecaf947" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquityComponentDomain_01a159ac-3da2-7332-98e0-e39b4ecaf947_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_EquityComponentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementEquityComponentsAxis_01a159ac-3da2-7332-98e0-e39b4ecaf947" xlink:to="loc_us-gaap_EquityComponentDomain_01a159ac-3da2-7332-98e0-e39b4ecaf947_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquityComponentDomain_01a159ac-3da2-7b28-b22d-c4fad6fa5546" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_EquityComponentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementEquityComponentsAxis_01a159ac-3da2-7332-98e0-e39b4ecaf947" xlink:to="loc_us-gaap_EquityComponentDomain_01a159ac-3da2-7b28-b22d-c4fad6fa5546" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockMember_01a159ac-3da2-7783-94d9-2eaefdd8240b" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CommonStockMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_01a159ac-3da2-7b28-b22d-c4fad6fa5546" xlink:to="loc_us-gaap_CommonStockMember_01a159ac-3da2-7783-94d9-2eaefdd8240b" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquityInterestIssuedOrIssuableByTypeAxis_01a159ac-3da2-73c4-96fa-f78f84cd3aba" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_EquityInterestIssuedOrIssuableByTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_ctos_ScheduleOfEquityTable_01a159ac-3da2-7552-b051-e71f680ab29c" xlink:to="loc_us-gaap_EquityInterestIssuedOrIssuableByTypeAxis_01a159ac-3da2-73c4-96fa-f78f84cd3aba" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquityInterestIssuedOrIssuableTypeDomain_01a159ac-3da2-73c4-96fa-f78f84cd3aba_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_EquityInterestIssuedOrIssuableTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_EquityInterestIssuedOrIssuableByTypeAxis_01a159ac-3da2-73c4-96fa-f78f84cd3aba" xlink:to="loc_us-gaap_EquityInterestIssuedOrIssuableTypeDomain_01a159ac-3da2-73c4-96fa-f78f84cd3aba_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquityInterestIssuedOrIssuableTypeDomain_01a159ac-3da2-7dcb-a840-fcbf401a3f22" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_EquityInterestIssuedOrIssuableTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_EquityInterestIssuedOrIssuableByTypeAxis_01a159ac-3da2-73c4-96fa-f78f84cd3aba" xlink:to="loc_us-gaap_EquityInterestIssuedOrIssuableTypeDomain_01a159ac-3da2-7dcb-a840-fcbf401a3f22" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockMember_01a159ac-3da2-7919-8979-cfa67d09eb1d" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CommonStockMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityInterestIssuedOrIssuableTypeDomain_01a159ac-3da2-7dcb-a840-fcbf401a3f22" xlink:to="loc_us-gaap_CommonStockMember_01a159ac-3da2-7919-8979-cfa67d09eb1d" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_ctos_EarnoutSharesAxis_01a159ac-3da2-7e07-99e3-2b9fc71f8718" xlink:href="ctos-20260930.xsd#ctos_EarnoutSharesAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_ctos_ScheduleOfEquityTable_01a159ac-3da2-7552-b051-e71f680ab29c" xlink:to="loc_ctos_EarnoutSharesAxis_01a159ac-3da2-7e07-99e3-2b9fc71f8718" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_ctos_EarnoutSharesDomain_01a159ac-3da2-7e07-99e3-2b9fc71f8718_default" xlink:href="ctos-20260930.xsd#ctos_EarnoutSharesDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_ctos_EarnoutSharesAxis_01a159ac-3da2-7e07-99e3-2b9fc71f8718" xlink:to="loc_ctos_EarnoutSharesDomain_01a159ac-3da2-7e07-99e3-2b9fc71f8718_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_ctos_EarnoutSharesDomain_01a159ac-3da2-7fb8-8d17-0bf301ae2267" xlink:href="ctos-20260930.xsd#ctos_EarnoutSharesDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_ctos_EarnoutSharesAxis_01a159ac-3da2-7e07-99e3-2b9fc71f8718" xlink:to="loc_ctos_EarnoutSharesDomain_01a159ac-3da2-7fb8-8d17-0bf301ae2267" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_ctos_MaximumTargetEarnoutSharesMember_01a159ac-3da2-7367-bdc2-4b17e927e953" xlink:href="ctos-20260930.xsd#ctos_MaximumTargetEarnoutSharesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_ctos_EarnoutSharesDomain_01a159ac-3da2-7fb8-8d17-0bf301ae2267" xlink:to="loc_ctos_MaximumTargetEarnoutSharesMember_01a159ac-3da2-7367-bdc2-4b17e927e953" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PreferredStockSharesAuthorized_01a159ac-3da2-7198-8192-9e05478f4e38" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_PreferredStockSharesAuthorized"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_ctos_EquityLineItems_01a159ac-3da2-7340-b903-cce9b994688c" xlink:to="loc_us-gaap_PreferredStockSharesAuthorized_01a159ac-3da2-7198-8192-9e05478f4e38" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PreferredStockParOrStatedValuePerShare_01a159ac-3da2-7a3f-884f-a321d1937cfd" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_PreferredStockParOrStatedValuePerShare"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_ctos_EquityLineItems_01a159ac-3da2-7340-b903-cce9b994688c" xlink:to="loc_us-gaap_PreferredStockParOrStatedValuePerShare_01a159ac-3da2-7a3f-884f-a321d1937cfd" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PreferredStockSharesIssued_01a159ac-3da2-7578-a293-d6164e72589d" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_PreferredStockSharesIssued"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_ctos_EquityLineItems_01a159ac-3da2-7340-b903-cce9b994688c" xlink:to="loc_us-gaap_PreferredStockSharesIssued_01a159ac-3da2-7578-a293-d6164e72589d" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PreferredStockSharesOutstanding_01a159ac-3da2-7d47-9125-219969a1e4fd" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_PreferredStockSharesOutstanding"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_ctos_EquityLineItems_01a159ac-3da2-7340-b903-cce9b994688c" xlink:to="loc_us-gaap_PreferredStockSharesOutstanding_01a159ac-3da2-7d47-9125-219969a1e4fd" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockSharesAuthorized_01a159ac-3da2-7818-9b73-56f21e2a0ef3" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CommonStockSharesAuthorized"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_ctos_EquityLineItems_01a159ac-3da2-7340-b903-cce9b994688c" xlink:to="loc_us-gaap_CommonStockSharesAuthorized_01a159ac-3da2-7818-9b73-56f21e2a0ef3" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockParOrStatedValuePerShare_01a159ac-3da2-7b90-b1a8-6b7502c868fb" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CommonStockParOrStatedValuePerShare"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_ctos_EquityLineItems_01a159ac-3da2-7340-b903-cce9b994688c" xlink:to="loc_us-gaap_CommonStockParOrStatedValuePerShare_01a159ac-3da2-7b90-b1a8-6b7502c868fb" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_StockRepurchaseProgramAuthorizedAmount1_01a159ac-3da2-7571-95d2-45be54aecc83" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_StockRepurchaseProgramAuthorizedAmount1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_ctos_EquityLineItems_01a159ac-3da2-7340-b903-cce9b994688c" xlink:to="loc_srt_StockRepurchaseProgramAuthorizedAmount1_01a159ac-3da2-7571-95d2-45be54aecc83" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_ctos_StockRepurchaseProgramAdditionalAuthorizedAmount_01a159ac-3da2-77ba-9458-546bb06c0786" xlink:href="ctos-20260930.xsd#ctos_StockRepurchaseProgramAdditionalAuthorizedAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_ctos_EquityLineItems_01a159ac-3da2-7340-b903-cce9b994688c" xlink:to="loc_ctos_StockRepurchaseProgramAdditionalAuthorizedAmount_01a159ac-3da2-77ba-9458-546bb06c0786" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockRepurchaseProgramRemainingAuthorizedRepurchaseAmount1_01a159ac-3da2-7fca-bdce-b6dbadfe225a" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StockRepurchaseProgramRemainingAuthorizedRepurchaseAmount1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_ctos_EquityLineItems_01a159ac-3da2-7340-b903-cce9b994688c" xlink:to="loc_us-gaap_StockRepurchaseProgramRemainingAuthorizedRepurchaseAmount1_01a159ac-3da2-7fca-bdce-b6dbadfe225a" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_ctos_EarnoutShares_01a159ac-3da2-7639-bf6c-a69880820669" xlink:href="ctos-20260930.xsd#ctos_EarnoutShares"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_ctos_EquityLineItems_01a159ac-3da2-7340-b903-cce9b994688c" xlink:to="loc_ctos_EarnoutShares_01a159ac-3da2-7639-bf6c-a69880820669" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_ctos_EarnoutSharesContingentConsiderationSharePrice_01a159ac-3da2-75ca-b2bf-d938138c5976" xlink:href="ctos-20260930.xsd#ctos_EarnoutSharesContingentConsiderationSharePrice"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_ctos_EquityLineItems_01a159ac-3da2-7340-b903-cce9b994688c" xlink:to="loc_ctos_EarnoutSharesContingentConsiderationSharePrice_01a159ac-3da2-75ca-b2bf-d938138c5976" xlink:type="arc" order="10"/>
    <link:loc xlink:type="locator" xlink:label="loc_ctos_EarnoutSharesContingentConsiderationThresholdTradingDays_01a159ac-3da2-7a67-8bfc-95ea4f87b85e" xlink:href="ctos-20260930.xsd#ctos_EarnoutSharesContingentConsiderationThresholdTradingDays"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_ctos_EquityLineItems_01a159ac-3da2-7340-b903-cce9b994688c" xlink:to="loc_ctos_EarnoutSharesContingentConsiderationThresholdTradingDays_01a159ac-3da2-7a67-8bfc-95ea4f87b85e" xlink:type="arc" order="11"/>
    <link:loc xlink:type="locator" xlink:label="loc_ctos_EarnoutSharesContingentConsiderationArrangementsThresholdConsecutiveTradingDays_01a159ac-3da2-7606-8dec-c1656a2b0376" xlink:href="ctos-20260930.xsd#ctos_EarnoutSharesContingentConsiderationArrangementsThresholdConsecutiveTradingDays"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_ctos_EquityLineItems_01a159ac-3da2-7340-b903-cce9b994688c" xlink:to="loc_ctos_EarnoutSharesContingentConsiderationArrangementsThresholdConsecutiveTradingDays_01a159ac-3da2-7606-8dec-c1656a2b0376" xlink:type="arc" order="12"/>
    <link:loc xlink:type="locator" xlink:label="loc_ctos_EarnoutSharesForfeitedDuringPeriodShares_01a159ac-3da2-7bb2-a326-41f8f82da83d" xlink:href="ctos-20260930.xsd#ctos_EarnoutSharesForfeitedDuringPeriodShares"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_ctos_EquityLineItems_01a159ac-3da2-7340-b903-cce9b994688c" xlink:to="loc_ctos_EarnoutSharesForfeitedDuringPeriodShares_01a159ac-3da2-7bb2-a326-41f8f82da83d" xlink:type="arc" order="13"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://customtruck.com/role/FairValueMeasurementsDetails" xlink:type="simple" xlink:href="ctos-20260930.xsd#FairValueMeasurementsDetails"/>
  <link:definitionLink xlink:role="http://customtruck.com/role/FairValueMeasurementsDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems_01a159ac-3da2-75ec-80ec-beeb9f83108b" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable_01a159ac-3da2-7c57-b42e-39934aa2326a" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems_01a159ac-3da2-75ec-80ec-beeb9f83108b" xlink:to="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable_01a159ac-3da2-7c57-b42e-39934aa2326a" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueByMeasurementBasisAxis_01a159ac-3da2-7158-9987-451072f47327" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueByMeasurementBasisAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable_01a159ac-3da2-7c57-b42e-39934aa2326a" xlink:to="loc_us-gaap_FairValueByMeasurementBasisAxis_01a159ac-3da2-7158-9987-451072f47327" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PortionAtFairValueFairValueDisclosureMember_01a159ac-3da2-7158-9987-451072f47327_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_PortionAtFairValueFairValueDisclosureMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FairValueByMeasurementBasisAxis_01a159ac-3da2-7158-9987-451072f47327" xlink:to="loc_us-gaap_PortionAtFairValueFairValueDisclosureMember_01a159ac-3da2-7158-9987-451072f47327_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueDisclosureItemAmountsDomain_01a159ac-3da2-7634-99e8-d53a14ed4bed" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueDisclosureItemAmountsDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FairValueByMeasurementBasisAxis_01a159ac-3da2-7158-9987-451072f47327" xlink:to="loc_us-gaap_FairValueDisclosureItemAmountsDomain_01a159ac-3da2-7634-99e8-d53a14ed4bed" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PortionAtFairValueFairValueDisclosureMember_01a159ac-3da2-7355-a706-c19aa20eb309" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_PortionAtFairValueFairValueDisclosureMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueDisclosureItemAmountsDomain_01a159ac-3da2-7634-99e8-d53a14ed4bed" xlink:to="loc_us-gaap_PortionAtFairValueFairValueDisclosureMember_01a159ac-3da2-7355-a706-c19aa20eb309" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CarryingReportedAmountFairValueDisclosureMember_01a159ac-3da2-7669-bda2-f562864cb261" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CarryingReportedAmountFairValueDisclosureMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PortionAtFairValueFairValueDisclosureMember_01a159ac-3da2-7355-a706-c19aa20eb309" xlink:to="loc_us-gaap_CarryingReportedAmountFairValueDisclosureMember_01a159ac-3da2-7669-bda2-f562864cb261" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EstimateOfFairValueFairValueDisclosureMember_01a159ac-3da2-7103-826b-98c187019917" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_EstimateOfFairValueFairValueDisclosureMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PortionAtFairValueFairValueDisclosureMember_01a159ac-3da2-7355-a706-c19aa20eb309" xlink:to="loc_us-gaap_EstimateOfFairValueFairValueDisclosureMember_01a159ac-3da2-7103-826b-98c187019917" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CreditFacilityAxis_01a159ac-3da2-7718-b622-1b9333eefb22" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CreditFacilityAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable_01a159ac-3da2-7c57-b42e-39934aa2326a" xlink:to="loc_us-gaap_CreditFacilityAxis_01a159ac-3da2-7718-b622-1b9333eefb22" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CreditFacilityDomain_01a159ac-3da2-7718-b622-1b9333eefb22_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CreditFacilityDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_CreditFacilityAxis_01a159ac-3da2-7718-b622-1b9333eefb22" xlink:to="loc_us-gaap_CreditFacilityDomain_01a159ac-3da2-7718-b622-1b9333eefb22_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CreditFacilityDomain_01a159ac-3da2-7ef1-97a0-dc970dd4b8fa" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CreditFacilityDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_CreditFacilityAxis_01a159ac-3da2-7718-b622-1b9333eefb22" xlink:to="loc_us-gaap_CreditFacilityDomain_01a159ac-3da2-7ef1-97a0-dc970dd4b8fa" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevolvingCreditFacilityMember_01a159ac-3da2-76e0-84d6-1962b0ce5767" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RevolvingCreditFacilityMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CreditFacilityDomain_01a159ac-3da2-7ef1-97a0-dc970dd4b8fa" xlink:to="loc_us-gaap_RevolvingCreditFacilityMember_01a159ac-3da2-76e0-84d6-1962b0ce5767" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentAxis_01a159ac-3da2-708a-b007-6c6658f7e228" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable_01a159ac-3da2-7c57-b42e-39934aa2326a" xlink:to="loc_us-gaap_DebtInstrumentAxis_01a159ac-3da2-708a-b007-6c6658f7e228" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_01a159ac-3da2-708a-b007-6c6658f7e228_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_DebtInstrumentAxis_01a159ac-3da2-708a-b007-6c6658f7e228" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_01a159ac-3da2-708a-b007-6c6658f7e228_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_01a159ac-3da2-7a84-a9fc-b020e38b890f" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_DebtInstrumentAxis_01a159ac-3da2-708a-b007-6c6658f7e228" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_01a159ac-3da2-7a84-a9fc-b020e38b890f" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_ctos_ABLFacilityMember_01a159ac-3da2-76ee-baf8-cf72ff23744a" xlink:href="ctos-20260930.xsd#ctos_ABLFacilityMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_01a159ac-3da2-7a84-a9fc-b020e38b890f" xlink:to="loc_ctos_ABLFacilityMember_01a159ac-3da2-76ee-baf8-cf72ff23744a" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_ctos_A550SeniorSecuredSecondLienNotesDue2029Member_01a159ac-3da2-70d4-9f47-7638d67049cc" xlink:href="ctos-20260930.xsd#ctos_A550SeniorSecuredSecondLienNotesDue2029Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_01a159ac-3da2-7a84-a9fc-b020e38b890f" xlink:to="loc_ctos_A550SeniorSecuredSecondLienNotesDue2029Member_01a159ac-3da2-70d4-9f47-7638d67049cc" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_ctos_A2023CreditFacilityMember_01a159ac-3da2-7f4b-acde-fe3ca34037cb" xlink:href="ctos-20260930.xsd#ctos_A2023CreditFacilityMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_01a159ac-3da2-7a84-a9fc-b020e38b890f" xlink:to="loc_ctos_A2023CreditFacilityMember_01a159ac-3da2-7f4b-acde-fe3ca34037cb" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeAxis_01a159ac-3da2-7d41-9194-e8e4747bbdb4" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LongtermDebtTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable_01a159ac-3da2-7c57-b42e-39934aa2326a" xlink:to="loc_us-gaap_LongtermDebtTypeAxis_01a159ac-3da2-7d41-9194-e8e4747bbdb4" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeDomain_01a159ac-3da2-7d41-9194-e8e4747bbdb4_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LongtermDebtTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_LongtermDebtTypeAxis_01a159ac-3da2-7d41-9194-e8e4747bbdb4" xlink:to="loc_us-gaap_LongtermDebtTypeDomain_01a159ac-3da2-7d41-9194-e8e4747bbdb4_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeDomain_01a159ac-3da2-718e-a76f-2d9a95d2a2fd" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LongtermDebtTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_LongtermDebtTypeAxis_01a159ac-3da2-7d41-9194-e8e4747bbdb4" xlink:to="loc_us-gaap_LongtermDebtTypeDomain_01a159ac-3da2-718e-a76f-2d9a95d2a2fd" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LineOfCreditMember_01a159ac-3da2-7144-a8a8-23e620573c4a" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LineOfCreditMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LongtermDebtTypeDomain_01a159ac-3da2-718e-a76f-2d9a95d2a2fd" xlink:to="loc_us-gaap_LineOfCreditMember_01a159ac-3da2-7144-a8a8-23e620573c4a" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SeniorNotesMember_01a159ac-3da2-7d0a-9e6f-d99850d7e143" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SeniorNotesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LongtermDebtTypeDomain_01a159ac-3da2-718e-a76f-2d9a95d2a2fd" xlink:to="loc_us-gaap_SeniorNotesMember_01a159ac-3da2-7d0a-9e6f-d99850d7e143" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NotesPayableOtherPayablesMember_01a159ac-3da2-713e-adda-6f83e394f3c4" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_NotesPayableOtherPayablesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LongtermDebtTypeDomain_01a159ac-3da2-718e-a76f-2d9a95d2a2fd" xlink:to="loc_us-gaap_NotesPayableOtherPayablesMember_01a159ac-3da2-713e-adda-6f83e394f3c4" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueByMeasurementFrequencyAxis_01a159ac-3da2-7b81-9a02-0e74ef464193" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueByMeasurementFrequencyAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable_01a159ac-3da2-7c57-b42e-39934aa2326a" xlink:to="loc_us-gaap_FairValueByMeasurementFrequencyAxis_01a159ac-3da2-7b81-9a02-0e74ef464193" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementFrequencyDomain_01a159ac-3da2-7b81-9a02-0e74ef464193_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueMeasurementFrequencyDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FairValueByMeasurementFrequencyAxis_01a159ac-3da2-7b81-9a02-0e74ef464193" xlink:to="loc_us-gaap_FairValueMeasurementFrequencyDomain_01a159ac-3da2-7b81-9a02-0e74ef464193_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementFrequencyDomain_01a159ac-3da2-7ac6-90db-4cfe1dbff8a6" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueMeasurementFrequencyDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FairValueByMeasurementFrequencyAxis_01a159ac-3da2-7b81-9a02-0e74ef464193" xlink:to="loc_us-gaap_FairValueMeasurementFrequencyDomain_01a159ac-3da2-7ac6-90db-4cfe1dbff8a6" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementsRecurringMember_01a159ac-3da2-77a6-8212-1cb407022265" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueMeasurementsRecurringMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueMeasurementFrequencyDomain_01a159ac-3da2-7ac6-90db-4cfe1dbff8a6" xlink:to="loc_us-gaap_FairValueMeasurementsRecurringMember_01a159ac-3da2-77a6-8212-1cb407022265" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_01a159ac-3da2-73a5-8ed4-98de69d7b596" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueByFairValueHierarchyLevelAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable_01a159ac-3da2-7c57-b42e-39934aa2326a" xlink:to="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_01a159ac-3da2-73a5-8ed4-98de69d7b596" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_01a159ac-3da2-73a5-8ed4-98de69d7b596_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueMeasurementsFairValueHierarchyDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_01a159ac-3da2-73a5-8ed4-98de69d7b596" xlink:to="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_01a159ac-3da2-73a5-8ed4-98de69d7b596_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_01a159ac-3da2-7543-8251-d9bc8d2c28d7" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueMeasurementsFairValueHierarchyDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_01a159ac-3da2-73a5-8ed4-98de69d7b596" xlink:to="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_01a159ac-3da2-7543-8251-d9bc8d2c28d7" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueInputsLevel1Member_01a159ac-3da2-738c-96fc-ab39f9b12280" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueInputsLevel1Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_01a159ac-3da2-7543-8251-d9bc8d2c28d7" xlink:to="loc_us-gaap_FairValueInputsLevel1Member_01a159ac-3da2-738c-96fc-ab39f9b12280" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueInputsLevel2Member_01a159ac-3da2-7677-be41-8dc952abaa47" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueInputsLevel2Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_01a159ac-3da2-7543-8251-d9bc8d2c28d7" xlink:to="loc_us-gaap_FairValueInputsLevel2Member_01a159ac-3da2-7677-be41-8dc952abaa47" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueInputsLevel3Member_01a159ac-3da2-758b-af2f-ef058143b751" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueInputsLevel3Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_01a159ac-3da2-7543-8251-d9bc8d2c28d7" xlink:to="loc_us-gaap_FairValueInputsLevel3Member_01a159ac-3da2-758b-af2f-ef058143b751" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongTermDebtFairValue_01a159ac-3da2-7301-8be4-c3833fefefc4" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LongTermDebtFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems_01a159ac-3da2-75ec-80ec-beeb9f83108b" xlink:to="loc_us-gaap_LongTermDebtFairValue_01a159ac-3da2-7301-8be4-c3833fefefc4" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://customtruck.com/role/CommitmentsandContingenciesDetails" xlink:type="simple" xlink:href="ctos-20260930.xsd#CommitmentsandContingenciesDetails"/>
  <link:definitionLink xlink:role="http://customtruck.com/role/CommitmentsandContingenciesDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LossContingenciesLineItems_01a159ac-3da2-7a05-8653-660dbcb74e07" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LossContingenciesLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LossContingenciesTable_01a159ac-3da2-76ea-b8e7-f130dfc555c5" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LossContingenciesTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_LossContingenciesLineItems_01a159ac-3da2-7a05-8653-660dbcb74e07" xlink:to="loc_us-gaap_LossContingenciesTable_01a159ac-3da2-76ea-b8e7-f130dfc555c5" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_LitigationCaseAxis_01a159ac-3da2-74c9-a010-253724bff2cc" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_LitigationCaseAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_LossContingenciesTable_01a159ac-3da2-76ea-b8e7-f130dfc555c5" xlink:to="loc_srt_LitigationCaseAxis_01a159ac-3da2-74c9-a010-253724bff2cc" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_LitigationCaseTypeDomain_01a159ac-3da2-74c9-a010-253724bff2cc_default" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_LitigationCaseTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_LitigationCaseAxis_01a159ac-3da2-74c9-a010-253724bff2cc" xlink:to="loc_srt_LitigationCaseTypeDomain_01a159ac-3da2-74c9-a010-253724bff2cc_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_LitigationCaseTypeDomain_01a159ac-3da2-797e-8540-e6fe130701b2" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_LitigationCaseTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_LitigationCaseAxis_01a159ac-3da2-74c9-a010-253724bff2cc" xlink:to="loc_srt_LitigationCaseTypeDomain_01a159ac-3da2-797e-8540-e6fe130701b2" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_ctos_CustomTruckLPMember_01a159ac-3da2-788b-8d02-732ce9733ca8" xlink:href="ctos-20260930.xsd#ctos_CustomTruckLPMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_LitigationCaseTypeDomain_01a159ac-3da2-797e-8540-e6fe130701b2" xlink:to="loc_ctos_CustomTruckLPMember_01a159ac-3da2-788b-8d02-732ce9733ca8" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_CounterpartyNameAxis_01a159ac-3da2-758c-b192-f1ec4fa65c6f" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_CounterpartyNameAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_LossContingenciesTable_01a159ac-3da2-76ea-b8e7-f130dfc555c5" xlink:to="loc_srt_CounterpartyNameAxis_01a159ac-3da2-758c-b192-f1ec4fa65c6f" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RepurchaseAgreementCounterpartyNameDomain_01a159ac-3da2-758c-b192-f1ec4fa65c6f_default" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_RepurchaseAgreementCounterpartyNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_CounterpartyNameAxis_01a159ac-3da2-758c-b192-f1ec4fa65c6f" xlink:to="loc_srt_RepurchaseAgreementCounterpartyNameDomain_01a159ac-3da2-758c-b192-f1ec4fa65c6f_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RepurchaseAgreementCounterpartyNameDomain_01a159ac-3da2-7916-8bea-e0c45aa6f7c6" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_RepurchaseAgreementCounterpartyNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_CounterpartyNameAxis_01a159ac-3da2-758c-b192-f1ec4fa65c6f" xlink:to="loc_srt_RepurchaseAgreementCounterpartyNameDomain_01a159ac-3da2-7916-8bea-e0c45aa6f7c6" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_ctos_BuyerMember_01a159ac-3da2-78a9-89da-a6b1e3a358c8" xlink:href="ctos-20260930.xsd#ctos_BuyerMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RepurchaseAgreementCounterpartyNameDomain_01a159ac-3da2-7916-8bea-e0c45aa6f7c6" xlink:to="loc_ctos_BuyerMember_01a159ac-3da2-78a9-89da-a6b1e3a358c8" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_LegalEntityAxis_01a159ac-3da2-7ead-9263-b9a612374ffa" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_LegalEntityAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_LossContingenciesTable_01a159ac-3da2-76ea-b8e7-f130dfc555c5" xlink:to="loc_dei_LegalEntityAxis_01a159ac-3da2-7ead-9263-b9a612374ffa" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityDomain_01a159ac-3da2-7ead-9263-b9a612374ffa_default" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_EntityDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_dei_LegalEntityAxis_01a159ac-3da2-7ead-9263-b9a612374ffa" xlink:to="loc_dei_EntityDomain_01a159ac-3da2-7ead-9263-b9a612374ffa_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityDomain_01a159ac-3da2-7e2b-9d7f-4ca81ea9cf5d" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_EntityDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_dei_LegalEntityAxis_01a159ac-3da2-7ead-9263-b9a612374ffa" xlink:to="loc_dei_EntityDomain_01a159ac-3da2-7e2b-9d7f-4ca81ea9cf5d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_ctos_SellersMember_01a159ac-3da2-7493-8619-220882cf827a" xlink:href="ctos-20260930.xsd#ctos_SellersMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_EntityDomain_01a159ac-3da2-7e2b-9d7f-4ca81ea9cf5d" xlink:to="loc_ctos_SellersMember_01a159ac-3da2-7493-8619-220882cf827a" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_ctos_LossContingencyTypeAxis_01a159ac-3da2-7b26-a8bf-5d4b74a35789" xlink:href="ctos-20260930.xsd#ctos_LossContingencyTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_LossContingenciesTable_01a159ac-3da2-76ea-b8e7-f130dfc555c5" xlink:to="loc_ctos_LossContingencyTypeAxis_01a159ac-3da2-7b26-a8bf-5d4b74a35789" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_ctos_LossContingencyTypeDomain_01a159ac-3da2-7b26-a8bf-5d4b74a35789_default" xlink:href="ctos-20260930.xsd#ctos_LossContingencyTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_ctos_LossContingencyTypeAxis_01a159ac-3da2-7b26-a8bf-5d4b74a35789" xlink:to="loc_ctos_LossContingencyTypeDomain_01a159ac-3da2-7b26-a8bf-5d4b74a35789_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_ctos_LossContingencyTypeDomain_01a159ac-3da2-7b38-b159-f8db04aa6a8a" xlink:href="ctos-20260930.xsd#ctos_LossContingencyTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_ctos_LossContingencyTypeAxis_01a159ac-3da2-7b26-a8bf-5d4b74a35789" xlink:to="loc_ctos_LossContingencyTypeDomain_01a159ac-3da2-7b38-b159-f8db04aa6a8a" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_ctos_LossesArisingOutOfBreachOfSellersPreClosingCovenantsMember_01a159ac-3da2-7817-b488-60fb4c0c52ca" xlink:href="ctos-20260930.xsd#ctos_LossesArisingOutOfBreachOfSellersPreClosingCovenantsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_ctos_LossContingencyTypeDomain_01a159ac-3da2-7b38-b159-f8db04aa6a8a" xlink:to="loc_ctos_LossesArisingOutOfBreachOfSellersPreClosingCovenantsMember_01a159ac-3da2-7817-b488-60fb4c0c52ca" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_ctos_LossesArisingOutOfCertainIndemnifiedTaxMattersMember_01a159ac-3da2-7910-ac14-a11226f99686" xlink:href="ctos-20260930.xsd#ctos_LossesArisingOutOfCertainIndemnifiedTaxMattersMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_ctos_LossContingencyTypeDomain_01a159ac-3da2-7b38-b159-f8db04aa6a8a" xlink:to="loc_ctos_LossesArisingOutOfCertainIndemnifiedTaxMattersMember_01a159ac-3da2-7910-ac14-a11226f99686" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TaxPeriodAxis_01a159ac-3da2-7bc9-a971-bc6c959aaba6" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_TaxPeriodAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_LossContingenciesTable_01a159ac-3da2-76ea-b8e7-f130dfc555c5" xlink:to="loc_us-gaap_TaxPeriodAxis_01a159ac-3da2-7bc9-a971-bc6c959aaba6" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TaxPeriodDomain_01a159ac-3da2-7bc9-a971-bc6c959aaba6_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_TaxPeriodDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_TaxPeriodAxis_01a159ac-3da2-7bc9-a971-bc6c959aaba6" xlink:to="loc_us-gaap_TaxPeriodDomain_01a159ac-3da2-7bc9-a971-bc6c959aaba6_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TaxPeriodDomain_01a159ac-3da2-713f-9931-ef307128dbff" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_TaxPeriodDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_TaxPeriodAxis_01a159ac-3da2-7bc9-a971-bc6c959aaba6" xlink:to="loc_us-gaap_TaxPeriodDomain_01a159ac-3da2-713f-9931-ef307128dbff" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TaxYear2015Member_01a159ac-3da2-7ac2-8f24-5ab4805327fa" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_TaxYear2015Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TaxPeriodDomain_01a159ac-3da2-713f-9931-ef307128dbff" xlink:to="loc_us-gaap_TaxYear2015Member_01a159ac-3da2-7ac2-8f24-5ab4805327fa" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LossContingencyEstimateOfPossibleLoss_01a159ac-3da2-7b7a-b191-50a39537359b" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LossContingencyEstimateOfPossibleLoss"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LossContingenciesLineItems_01a159ac-3da2-7a05-8653-660dbcb74e07" xlink:to="loc_us-gaap_LossContingencyEstimateOfPossibleLoss_01a159ac-3da2-7b7a-b191-50a39537359b" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_ctos_IncomeTaxExaminationAssessment_01a159ac-3da2-74d2-a496-593b56bf82ec" xlink:href="ctos-20260930.xsd#ctos_IncomeTaxExaminationAssessment"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LossContingenciesLineItems_01a159ac-3da2-7a05-8653-660dbcb74e07" xlink:to="loc_ctos_IncomeTaxExaminationAssessment_01a159ac-3da2-74d2-a496-593b56bf82ec" xlink:type="arc" order="1"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://customtruck.com/role/RelatedPartiesScheduleofRelatedPartyTransactionsinCondensedConsolidatedStatementsofOperationsandComprehensiveIncomeLossDetails" xlink:type="simple" xlink:href="ctos-20260930.xsd#RelatedPartiesScheduleofRelatedPartyTransactionsinCondensedConsolidatedStatementsofOperationsandComprehensiveIncomeLossDetails"/>
  <link:definitionLink xlink:role="http://customtruck.com/role/RelatedPartiesScheduleofRelatedPartyTransactionsinCondensedConsolidatedStatementsofOperationsandComprehensiveIncomeLossDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RelatedPartyTransactionLineItems_01a159ac-3da2-7c91-9b7e-02904554e75a" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RelatedPartyTransactionLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfRelatedPartyTransactionsByRelatedPartyTable_01a159ac-3da2-7912-a6b7-230f0229996f" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ScheduleOfRelatedPartyTransactionsByRelatedPartyTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_RelatedPartyTransactionLineItems_01a159ac-3da2-7c91-9b7e-02904554e75a" xlink:to="loc_us-gaap_ScheduleOfRelatedPartyTransactionsByRelatedPartyTable_01a159ac-3da2-7912-a6b7-230f0229996f" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RelatedPartyTransactionsByRelatedPartyAxis_01a159ac-3da2-7af8-8939-6b36195a5461" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RelatedPartyTransactionsByRelatedPartyAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfRelatedPartyTransactionsByRelatedPartyTable_01a159ac-3da2-7912-a6b7-230f0229996f" xlink:to="loc_us-gaap_RelatedPartyTransactionsByRelatedPartyAxis_01a159ac-3da2-7af8-8939-6b36195a5461" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RelatedPartyDomain_01a159ac-3da2-7af8-8939-6b36195a5461_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RelatedPartyDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_RelatedPartyTransactionsByRelatedPartyAxis_01a159ac-3da2-7af8-8939-6b36195a5461" xlink:to="loc_us-gaap_RelatedPartyDomain_01a159ac-3da2-7af8-8939-6b36195a5461_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RelatedPartyDomain_01a159ac-3da2-773e-9eb3-6a7b57c9f492" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RelatedPartyDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_RelatedPartyTransactionsByRelatedPartyAxis_01a159ac-3da2-7af8-8939-6b36195a5461" xlink:to="loc_us-gaap_RelatedPartyDomain_01a159ac-3da2-773e-9eb3-6a7b57c9f492" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RelatedPartyMember_01a159ac-3da2-78e1-ab9f-af602fb1afd1" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RelatedPartyMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RelatedPartyDomain_01a159ac-3da2-773e-9eb3-6a7b57c9f492" xlink:to="loc_us-gaap_RelatedPartyMember_01a159ac-3da2-78e1-ab9f-af602fb1afd1" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_Revenues_01a159ac-3da2-7e71-893c-3b7e75efcba8" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_Revenues"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RelatedPartyTransactionLineItems_01a159ac-3da2-7c91-9b7e-02904554e75a" xlink:to="loc_us-gaap_Revenues_01a159ac-3da2-7e71-893c-3b7e75efcba8" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CostOfRevenue_01a159ac-3da2-7709-9b27-6ba44d7b6bf6" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CostOfRevenue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RelatedPartyTransactionLineItems_01a159ac-3da2-7c91-9b7e-02904554e75a" xlink:to="loc_us-gaap_CostOfRevenue_01a159ac-3da2-7709-9b27-6ba44d7b6bf6" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OperatingExpenses_01a159ac-3da2-713e-98ee-2815e4d7c8f7" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_OperatingExpenses"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RelatedPartyTransactionLineItems_01a159ac-3da2-7c91-9b7e-02904554e75a" xlink:to="loc_us-gaap_OperatingExpenses_01a159ac-3da2-713e-98ee-2815e4d7c8f7" xlink:type="arc" order="2"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://customtruck.com/role/RelatedPartiesScheduleofRelatedPartyTransactionsinCondensedConsolidatedBalanceSheetsDetails" xlink:type="simple" xlink:href="ctos-20260930.xsd#RelatedPartiesScheduleofRelatedPartyTransactionsinCondensedConsolidatedBalanceSheetsDetails"/>
  <link:definitionLink xlink:role="http://customtruck.com/role/RelatedPartiesScheduleofRelatedPartyTransactionsinCondensedConsolidatedBalanceSheetsDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RelatedPartyTransactionLineItems_01a159ac-3da2-7024-8b60-c67bb1ba1b72" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RelatedPartyTransactionLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfRelatedPartyTransactionsByRelatedPartyTable_01a159ac-3da2-72d6-aa72-61afd16d4267" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ScheduleOfRelatedPartyTransactionsByRelatedPartyTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_RelatedPartyTransactionLineItems_01a159ac-3da2-7024-8b60-c67bb1ba1b72" xlink:to="loc_us-gaap_ScheduleOfRelatedPartyTransactionsByRelatedPartyTable_01a159ac-3da2-72d6-aa72-61afd16d4267" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RelatedPartyTransactionsByRelatedPartyAxis_01a159ac-3da2-7865-8c30-d8782ab3ce93" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RelatedPartyTransactionsByRelatedPartyAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfRelatedPartyTransactionsByRelatedPartyTable_01a159ac-3da2-72d6-aa72-61afd16d4267" xlink:to="loc_us-gaap_RelatedPartyTransactionsByRelatedPartyAxis_01a159ac-3da2-7865-8c30-d8782ab3ce93" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RelatedPartyDomain_01a159ac-3da2-7865-8c30-d8782ab3ce93_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RelatedPartyDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_RelatedPartyTransactionsByRelatedPartyAxis_01a159ac-3da2-7865-8c30-d8782ab3ce93" xlink:to="loc_us-gaap_RelatedPartyDomain_01a159ac-3da2-7865-8c30-d8782ab3ce93_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RelatedPartyDomain_01a159ac-3da2-795c-91ac-8dd57cf8dcbb" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RelatedPartyDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_RelatedPartyTransactionsByRelatedPartyAxis_01a159ac-3da2-7865-8c30-d8782ab3ce93" xlink:to="loc_us-gaap_RelatedPartyDomain_01a159ac-3da2-795c-91ac-8dd57cf8dcbb" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RelatedPartyMember_01a159ac-3da2-7de0-a0ea-b48d32c972d7" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RelatedPartyMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RelatedPartyDomain_01a159ac-3da2-795c-91ac-8dd57cf8dcbb" xlink:to="loc_us-gaap_RelatedPartyMember_01a159ac-3da2-7de0-a0ea-b48d32c972d7" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccountsReceivableNetCurrent_01a159ac-3da2-7e2d-bfab-a7331dc1e818" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AccountsReceivableNetCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RelatedPartyTransactionLineItems_01a159ac-3da2-7024-8b60-c67bb1ba1b72" xlink:to="loc_us-gaap_AccountsReceivableNetCurrent_01a159ac-3da2-7e2d-bfab-a7331dc1e818" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccountsPayableCurrent_01a159ac-3da2-7dd0-be2f-4ecb9088ef1b" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AccountsPayableCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RelatedPartyTransactionLineItems_01a159ac-3da2-7024-8b60-c67bb1ba1b72" xlink:to="loc_us-gaap_AccountsPayableCurrent_01a159ac-3da2-7dd0-be2f-4ecb9088ef1b" xlink:type="arc" order="1"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://customtruck.com/role/SegmentsScheduleofCompanysSegmentResultsDetails" xlink:type="simple" xlink:href="ctos-20260930.xsd#SegmentsScheduleofCompanysSegmentResultsDetails"/>
  <link:definitionLink xlink:role="http://customtruck.com/role/SegmentsScheduleofCompanysSegmentResultsDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentReportingInformationLineItems_01a159ac-3da2-7c48-88ad-666af8daabb0" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SegmentReportingInformationLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable_01a159ac-3da2-7c3b-9915-ccf855ef0acc" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_01a159ac-3da2-7c48-88ad-666af8daabb0" xlink:to="loc_us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable_01a159ac-3da2-7c3b-9915-ccf855ef0acc" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementBusinessSegmentsAxis_01a159ac-3da2-7319-8ea3-3cd30b04a8f0" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StatementBusinessSegmentsAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable_01a159ac-3da2-7c3b-9915-ccf855ef0acc" xlink:to="loc_us-gaap_StatementBusinessSegmentsAxis_01a159ac-3da2-7319-8ea3-3cd30b04a8f0" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentDomain_01a159ac-3da2-7319-8ea3-3cd30b04a8f0_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SegmentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementBusinessSegmentsAxis_01a159ac-3da2-7319-8ea3-3cd30b04a8f0" xlink:to="loc_us-gaap_SegmentDomain_01a159ac-3da2-7319-8ea3-3cd30b04a8f0_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentDomain_01a159ac-3da2-73b9-a451-df27c4be3af5" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SegmentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementBusinessSegmentsAxis_01a159ac-3da2-7319-8ea3-3cd30b04a8f0" xlink:to="loc_us-gaap_SegmentDomain_01a159ac-3da2-73b9-a451-df27c4be3af5" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_ctos_SpecialtyEquipmentRentalsSegmentMember_01a159ac-3da2-7150-becb-3cb851986780" xlink:href="ctos-20260930.xsd#ctos_SpecialtyEquipmentRentalsSegmentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentDomain_01a159ac-3da2-73b9-a451-df27c4be3af5" xlink:to="loc_ctos_SpecialtyEquipmentRentalsSegmentMember_01a159ac-3da2-7150-becb-3cb851986780" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_ctos_SpecialtyTruckEquipmentAndManufacturingSegmentMember_01a159ac-3da2-7c86-b9fe-4425c0b55e50" xlink:href="ctos-20260930.xsd#ctos_SpecialtyTruckEquipmentAndManufacturingSegmentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentDomain_01a159ac-3da2-73b9-a451-df27c4be3af5" xlink:to="loc_ctos_SpecialtyTruckEquipmentAndManufacturingSegmentMember_01a159ac-3da2-7c86-b9fe-4425c0b55e50" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ProductOrServiceAxis_01a159ac-3da2-7c53-b166-1fabb9f6e485" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_ProductOrServiceAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable_01a159ac-3da2-7c3b-9915-ccf855ef0acc" xlink:to="loc_srt_ProductOrServiceAxis_01a159ac-3da2-7c53-b166-1fabb9f6e485" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ProductsAndServicesDomain_01a159ac-3da2-7c53-b166-1fabb9f6e485_default" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_ProductsAndServicesDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_ProductOrServiceAxis_01a159ac-3da2-7c53-b166-1fabb9f6e485" xlink:to="loc_srt_ProductsAndServicesDomain_01a159ac-3da2-7c53-b166-1fabb9f6e485_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ProductsAndServicesDomain_01a159ac-3da2-7507-b8c7-6ff9514c211c" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_ProductsAndServicesDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_ProductOrServiceAxis_01a159ac-3da2-7c53-b166-1fabb9f6e485" xlink:to="loc_srt_ProductsAndServicesDomain_01a159ac-3da2-7507-b8c7-6ff9514c211c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_ctos_RentalMember_01a159ac-3da2-7ab0-9f92-bd82169a5236" xlink:href="ctos-20260930.xsd#ctos_RentalMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ProductsAndServicesDomain_01a159ac-3da2-7507-b8c7-6ff9514c211c" xlink:to="loc_ctos_RentalMember_01a159ac-3da2-7ab0-9f92-bd82169a5236" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_ctos_EquipmentSalesMember_01a159ac-3da2-7762-ac0a-ed37c3ce07b7" xlink:href="ctos-20260930.xsd#ctos_EquipmentSalesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ProductsAndServicesDomain_01a159ac-3da2-7507-b8c7-6ff9514c211c" xlink:to="loc_ctos_EquipmentSalesMember_01a159ac-3da2-7762-ac0a-ed37c3ce07b7" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_ctos_PartsSalesAndServicesMember_01a159ac-3da2-7508-883e-24a59382c8ff" xlink:href="ctos-20260930.xsd#ctos_PartsSalesAndServicesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ProductsAndServicesDomain_01a159ac-3da2-7507-b8c7-6ff9514c211c" xlink:to="loc_ctos_PartsSalesAndServicesMember_01a159ac-3da2-7508-883e-24a59382c8ff" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_ctos_OtherSegmentItemsAxis_01a159ac-3da2-7ad1-9f3f-9973eabe59c0" xlink:href="ctos-20260930.xsd#ctos_OtherSegmentItemsAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable_01a159ac-3da2-7c3b-9915-ccf855ef0acc" xlink:to="loc_ctos_OtherSegmentItemsAxis_01a159ac-3da2-7ad1-9f3f-9973eabe59c0" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_ctos_OtherSegmentItemsDomain_01a159ac-3da2-7ad1-9f3f-9973eabe59c0_default" xlink:href="ctos-20260930.xsd#ctos_OtherSegmentItemsDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_ctos_OtherSegmentItemsAxis_01a159ac-3da2-7ad1-9f3f-9973eabe59c0" xlink:to="loc_ctos_OtherSegmentItemsDomain_01a159ac-3da2-7ad1-9f3f-9973eabe59c0_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_ctos_OtherSegmentItemsDomain_01a159ac-3da2-7f88-9752-7f027bc0e691" xlink:href="ctos-20260930.xsd#ctos_OtherSegmentItemsDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_ctos_OtherSegmentItemsAxis_01a159ac-3da2-7ad1-9f3f-9973eabe59c0" xlink:to="loc_ctos_OtherSegmentItemsDomain_01a159ac-3da2-7f88-9752-7f027bc0e691" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_ctos_IntersegmentTransactionsMember_01a159ac-3da2-73de-8308-2f3decf94d5e" xlink:href="ctos-20260930.xsd#ctos_IntersegmentTransactionsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_ctos_OtherSegmentItemsDomain_01a159ac-3da2-7f88-9752-7f027bc0e691" xlink:to="loc_ctos_IntersegmentTransactionsMember_01a159ac-3da2-73de-8308-2f3decf94d5e" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_ctos_RentalARProvisionMember_01a159ac-3da2-7bf8-af8d-bd4fe4e6f459" xlink:href="ctos-20260930.xsd#ctos_RentalARProvisionMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_ctos_OtherSegmentItemsDomain_01a159ac-3da2-7f88-9752-7f027bc0e691" xlink:to="loc_ctos_RentalARProvisionMember_01a159ac-3da2-7bf8-af8d-bd4fe4e6f459" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_ctos_SalesTypeLeaseAdjustmentMember_01a159ac-3da2-7d93-b146-bae266628491" xlink:href="ctos-20260930.xsd#ctos_SalesTypeLeaseAdjustmentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_ctos_OtherSegmentItemsDomain_01a159ac-3da2-7f88-9752-7f027bc0e691" xlink:to="loc_ctos_SalesTypeLeaseAdjustmentMember_01a159ac-3da2-7d93-b146-bae266628491" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentAxis_01a159ac-3da2-7475-b65e-2ea32b5698ed" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable_01a159ac-3da2-7c3b-9915-ccf855ef0acc" xlink:to="loc_us-gaap_DebtInstrumentAxis_01a159ac-3da2-7475-b65e-2ea32b5698ed" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_01a159ac-3da2-7475-b65e-2ea32b5698ed_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_DebtInstrumentAxis_01a159ac-3da2-7475-b65e-2ea32b5698ed" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_01a159ac-3da2-7475-b65e-2ea32b5698ed_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_01a159ac-3da2-7f36-ab2a-f63ac2cd6e02" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_DebtInstrumentAxis_01a159ac-3da2-7475-b65e-2ea32b5698ed" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_01a159ac-3da2-7f36-ab2a-f63ac2cd6e02" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_ctos_FloorPlanFinancingArrangementsMember_01a159ac-3da2-7a88-8a72-eb81a7fda20e" xlink:href="ctos-20260930.xsd#ctos_FloorPlanFinancingArrangementsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_01a159ac-3da2-7f36-ab2a-f63ac2cd6e02" xlink:to="loc_ctos_FloorPlanFinancingArrangementsMember_01a159ac-3da2-7a88-8a72-eb81a7fda20e" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_Revenues_01a159ac-3da2-71fc-af02-661311aea10f" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_Revenues"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_01a159ac-3da2-7c48-88ad-666af8daabb0" xlink:to="loc_us-gaap_Revenues_01a159ac-3da2-71fc-af02-661311aea10f" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_ctos_RevenuesOtherSegmentItems_01a159ac-3da2-715f-af9c-ffd6bbd76a78" xlink:href="ctos-20260930.xsd#ctos_RevenuesOtherSegmentItems"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_01a159ac-3da2-7c48-88ad-666af8daabb0" xlink:to="loc_ctos_RevenuesOtherSegmentItems_01a159ac-3da2-715f-af9c-ffd6bbd76a78" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_ctos_TotalSegmentRevenue_01a159ac-3da2-7127-8a22-cd652271ebed" xlink:href="ctos-20260930.xsd#ctos_TotalSegmentRevenue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_01a159ac-3da2-7c48-88ad-666af8daabb0" xlink:to="loc_ctos_TotalSegmentRevenue_01a159ac-3da2-7127-8a22-cd652271ebed" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_ctos_SegmentCostOfGoodsAndServicesExcludingDepreciation_01a159ac-3da2-7f9d-acfe-376a9340becb" xlink:href="ctos-20260930.xsd#ctos_SegmentCostOfGoodsAndServicesExcludingDepreciation"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_01a159ac-3da2-7c48-88ad-666af8daabb0" xlink:to="loc_ctos_SegmentCostOfGoodsAndServicesExcludingDepreciation_01a159ac-3da2-7f9d-acfe-376a9340becb" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_ctos_CostOfEquipmentSalesNetOfPurchaseAccountingSalesTypeLeasesAndDepreciation_01a159ac-3da2-7b9f-9cf8-fd039e68c2c3" xlink:href="ctos-20260930.xsd#ctos_CostOfEquipmentSalesNetOfPurchaseAccountingSalesTypeLeasesAndDepreciation"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_01a159ac-3da2-7c48-88ad-666af8daabb0" xlink:to="loc_ctos_CostOfEquipmentSalesNetOfPurchaseAccountingSalesTypeLeasesAndDepreciation_01a159ac-3da2-7b9f-9cf8-fd039e68c2c3" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_ctos_CostOfGoodsAndServiceOtherSegmentItems_01a159ac-3da2-7b6f-a2a6-e09906eba2ed" xlink:href="ctos-20260930.xsd#ctos_CostOfGoodsAndServiceOtherSegmentItems"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_01a159ac-3da2-7c48-88ad-666af8daabb0" xlink:to="loc_ctos_CostOfGoodsAndServiceOtherSegmentItems_01a159ac-3da2-7b6f-a2a6-e09906eba2ed" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_ctos_TotalCostOfSegmentRevenue_01a159ac-3da2-7db6-a910-4a1ec6e13aa4" xlink:href="ctos-20260930.xsd#ctos_TotalCostOfSegmentRevenue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_01a159ac-3da2-7c48-88ad-666af8daabb0" xlink:to="loc_ctos_TotalCostOfSegmentRevenue_01a159ac-3da2-7db6-a910-4a1ec6e13aa4" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_ctos_SegmentSellingGeneralAndAdministrativeExpense_01a159ac-3da2-783c-9b7d-ff911c65e49a" xlink:href="ctos-20260930.xsd#ctos_SegmentSellingGeneralAndAdministrativeExpense"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_01a159ac-3da2-7c48-88ad-666af8daabb0" xlink:to="loc_ctos_SegmentSellingGeneralAndAdministrativeExpense_01a159ac-3da2-783c-9b7d-ff911c65e49a" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_ctos_FloorplanInterestExpense_01a159ac-3da2-7c58-afcc-1a73ad6b3e64" xlink:href="ctos-20260930.xsd#ctos_FloorplanInterestExpense"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_01a159ac-3da2-7c48-88ad-666af8daabb0" xlink:to="loc_ctos_FloorplanInterestExpense_01a159ac-3da2-7c58-afcc-1a73ad6b3e64" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_ctos_TotalSegmentExpenses_01a159ac-3da2-7922-b7bb-de3f0c2bd1cd" xlink:href="ctos-20260930.xsd#ctos_TotalSegmentExpenses"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_01a159ac-3da2-7c48-88ad-666af8daabb0" xlink:to="loc_ctos_TotalSegmentExpenses_01a159ac-3da2-7922-b7bb-de3f0c2bd1cd" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_ctos_SegmentAdjustedEarningsBeforeInterestTaxesDepreciationAndAmortization_01a159ac-3da2-7f6e-985f-05557ecb85e0" xlink:href="ctos-20260930.xsd#ctos_SegmentAdjustedEarningsBeforeInterestTaxesDepreciationAndAmortization"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_01a159ac-3da2-7c48-88ad-666af8daabb0" xlink:to="loc_ctos_SegmentAdjustedEarningsBeforeInterestTaxesDepreciationAndAmortization_01a159ac-3da2-7f6e-985f-05557ecb85e0" xlink:type="arc" order="10"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://customtruck.com/role/SegmentsScheduleofReconciliationofSegmentsAdjustedEarningsBeforeInterestTaxesDepreciationandAmortizationAdjustedEBITDAtoConsolidatedIncomeLossBeforeIncomeTaxesDetails" xlink:type="simple" xlink:href="ctos-20260930.xsd#SegmentsScheduleofReconciliationofSegmentsAdjustedEarningsBeforeInterestTaxesDepreciationandAmortizationAdjustedEBITDAtoConsolidatedIncomeLossBeforeIncomeTaxesDetails"/>
  <link:definitionLink xlink:role="http://customtruck.com/role/SegmentsScheduleofReconciliationofSegmentsAdjustedEarningsBeforeInterestTaxesDepreciationandAmortizationAdjustedEBITDAtoConsolidatedIncomeLossBeforeIncomeTaxesDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentReportingInformationLineItems_01a159ac-3da2-7d78-abc5-b3b002f828f4" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SegmentReportingInformationLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable_01a159ac-3da2-7b8f-a486-149af8d7dcb9" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_01a159ac-3da2-7d78-abc5-b3b002f828f4" xlink:to="loc_us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable_01a159ac-3da2-7b8f-a486-149af8d7dcb9" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementBusinessSegmentsAxis_01a159ac-3da2-7859-830d-8a201315a1c8" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StatementBusinessSegmentsAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable_01a159ac-3da2-7b8f-a486-149af8d7dcb9" xlink:to="loc_us-gaap_StatementBusinessSegmentsAxis_01a159ac-3da2-7859-830d-8a201315a1c8" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentDomain_01a159ac-3da2-7859-830d-8a201315a1c8_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SegmentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementBusinessSegmentsAxis_01a159ac-3da2-7859-830d-8a201315a1c8" xlink:to="loc_us-gaap_SegmentDomain_01a159ac-3da2-7859-830d-8a201315a1c8_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentDomain_01a159ac-3da2-73e3-a781-d722dcc62973" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SegmentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementBusinessSegmentsAxis_01a159ac-3da2-7859-830d-8a201315a1c8" xlink:to="loc_us-gaap_SegmentDomain_01a159ac-3da2-73e3-a781-d722dcc62973" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_ctos_SpecialtyEquipmentRentalsSegmentMember_01a159ac-3da2-74d3-94fe-2033620fc42a" xlink:href="ctos-20260930.xsd#ctos_SpecialtyEquipmentRentalsSegmentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentDomain_01a159ac-3da2-73e3-a781-d722dcc62973" xlink:to="loc_ctos_SpecialtyEquipmentRentalsSegmentMember_01a159ac-3da2-74d3-94fe-2033620fc42a" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_ctos_SpecialtyTruckEquipmentAndManufacturingSegmentMember_01a159ac-3da2-7066-8da8-5cb91b462b87" xlink:href="ctos-20260930.xsd#ctos_SpecialtyTruckEquipmentAndManufacturingSegmentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentDomain_01a159ac-3da2-73e3-a781-d722dcc62973" xlink:to="loc_ctos_SpecialtyTruckEquipmentAndManufacturingSegmentMember_01a159ac-3da2-7066-8da8-5cb91b462b87" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_ctos_SegmentAdjustedEarningsBeforeInterestTaxesDepreciationAndAmortization_01a159ac-3da2-735a-808c-63b262cd9eee" xlink:href="ctos-20260930.xsd#ctos_SegmentAdjustedEarningsBeforeInterestTaxesDepreciationAndAmortization"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_01a159ac-3da2-7d78-abc5-b3b002f828f4" xlink:to="loc_ctos_SegmentAdjustedEarningsBeforeInterestTaxesDepreciationAndAmortization_01a159ac-3da2-735a-808c-63b262cd9eee" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_ctos_IntersegmentMargin_01a159ac-3da2-7919-a46d-5138ea9b8577" xlink:href="ctos-20260930.xsd#ctos_IntersegmentMargin"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_01a159ac-3da2-7d78-abc5-b3b002f828f4" xlink:to="loc_ctos_IntersegmentMargin_01a159ac-3da2-7919-a46d-5138ea9b8577" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_ctos_CorporateAndNonAllocatedSellingGeneralAndAdministrativeExpenses_01a159ac-3da2-70a2-8f09-0d086b045d0c" xlink:href="ctos-20260930.xsd#ctos_CorporateAndNonAllocatedSellingGeneralAndAdministrativeExpenses"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_01a159ac-3da2-7d78-abc5-b3b002f828f4" xlink:to="loc_ctos_CorporateAndNonAllocatedSellingGeneralAndAdministrativeExpenses_01a159ac-3da2-70a2-8f09-0d086b045d0c" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_ctos_DepreciationAndAmortizationReconciliationOfAdjustedEBITDAToConsolidatedIncomeLossBeforeTaxes_01a159ac-3da2-7ad4-8ec2-c519bd8647a7" xlink:href="ctos-20260930.xsd#ctos_DepreciationAndAmortizationReconciliationOfAdjustedEBITDAToConsolidatedIncomeLossBeforeTaxes"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_01a159ac-3da2-7d78-abc5-b3b002f828f4" xlink:to="loc_ctos_DepreciationAndAmortizationReconciliationOfAdjustedEBITDAToConsolidatedIncomeLossBeforeTaxes_01a159ac-3da2-7ad4-8ec2-c519bd8647a7" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_ctos_InterestExpenseNetNonFloorplan_01a159ac-3da2-723b-b0c1-df3f89a6f5a6" xlink:href="ctos-20260930.xsd#ctos_InterestExpenseNetNonFloorplan"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_01a159ac-3da2-7d78-abc5-b3b002f828f4" xlink:to="loc_ctos_InterestExpenseNetNonFloorplan_01a159ac-3da2-723b-b0c1-df3f89a6f5a6" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_ctos_NonCashPurchaseAccountingImpact_01a159ac-3da2-77eb-b4e6-1d1031006306" xlink:href="ctos-20260930.xsd#ctos_NonCashPurchaseAccountingImpact"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_01a159ac-3da2-7d78-abc5-b3b002f828f4" xlink:to="loc_ctos_NonCashPurchaseAccountingImpact_01a159ac-3da2-77eb-b4e6-1d1031006306" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_ctos_MergerRelatedCosts_01a159ac-3da2-73f9-9222-3b3e8cd93f69" xlink:href="ctos-20260930.xsd#ctos_MergerRelatedCosts"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_01a159ac-3da2-7d78-abc5-b3b002f828f4" xlink:to="loc_ctos_MergerRelatedCosts_01a159ac-3da2-73f9-9222-3b3e8cd93f69" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_ctos_SalesTypeLeaseAdjustment_01a159ac-3da2-76ef-ae69-dcd7858ef5da" xlink:href="ctos-20260930.xsd#ctos_SalesTypeLeaseAdjustment"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_01a159ac-3da2-7d78-abc5-b3b002f828f4" xlink:to="loc_ctos_SalesTypeLeaseAdjustment_01a159ac-3da2-76ef-ae69-dcd7858ef5da" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AllocatedShareBasedCompensationExpense_01a159ac-3da2-7fe8-8956-5d0d3faec70d" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AllocatedShareBasedCompensationExpense"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_01a159ac-3da2-7d78-abc5-b3b002f828f4" xlink:to="loc_us-gaap_AllocatedShareBasedCompensationExpense_01a159ac-3da2-7fe8-8956-5d0d3faec70d" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeLossFromContinuingOperationsBeforeIncomeTaxesExtraordinaryItemsNoncontrollingInterest_01a159ac-3da2-7158-a802-c013a9d58591" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_IncomeLossFromContinuingOperationsBeforeIncomeTaxesExtraordinaryItemsNoncontrollingInterest"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_01a159ac-3da2-7d78-abc5-b3b002f828f4" xlink:to="loc_us-gaap_IncomeLossFromContinuingOperationsBeforeIncomeTaxesExtraordinaryItemsNoncontrollingInterest_01a159ac-3da2-7158-a802-c013a9d58591" xlink:type="arc" order="9"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://customtruck.com/role/SegmentsScheduleofTotalAssetsbyCountryDetails" xlink:type="simple" xlink:href="ctos-20260930.xsd#SegmentsScheduleofTotalAssetsbyCountryDetails"/>
  <link:definitionLink xlink:role="http://customtruck.com/role/SegmentsScheduleofTotalAssetsbyCountryDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentReportingInformationLineItems_01a159ac-3da2-78c6-a563-fe45912dfb36" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SegmentReportingInformationLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable_01a159ac-3da2-7f8f-b743-e2c59a34165f" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_01a159ac-3da2-78c6-a563-fe45912dfb36" xlink:to="loc_us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable_01a159ac-3da2-7f8f-b743-e2c59a34165f" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_StatementGeographicalAxis_01a159ac-3da2-775e-b7ad-0fcc77be1153" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_StatementGeographicalAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable_01a159ac-3da2-7f8f-b743-e2c59a34165f" xlink:to="loc_srt_StatementGeographicalAxis_01a159ac-3da2-775e-b7ad-0fcc77be1153" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_SegmentGeographicalDomain_01a159ac-3da2-775e-b7ad-0fcc77be1153_default" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_SegmentGeographicalDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_StatementGeographicalAxis_01a159ac-3da2-775e-b7ad-0fcc77be1153" xlink:to="loc_srt_SegmentGeographicalDomain_01a159ac-3da2-775e-b7ad-0fcc77be1153_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_SegmentGeographicalDomain_01a159ac-3da2-7422-a883-198383ba0e56" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_SegmentGeographicalDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_StatementGeographicalAxis_01a159ac-3da2-775e-b7ad-0fcc77be1153" xlink:to="loc_srt_SegmentGeographicalDomain_01a159ac-3da2-7422-a883-198383ba0e56" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_country_US_01a159ac-3da2-7720-9a06-c7092062a08d" xlink:href="https://xbrl.sec.gov/country/2026/country-2026.xsd#country_US"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_SegmentGeographicalDomain_01a159ac-3da2-7422-a883-198383ba0e56" xlink:to="loc_country_US_01a159ac-3da2-7720-9a06-c7092062a08d" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_country_CA_01a159ac-3da2-799d-925f-2bc262ddf895" xlink:href="https://xbrl.sec.gov/country/2026/country-2026.xsd#country_CA"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_SegmentGeographicalDomain_01a159ac-3da2-7422-a883-198383ba0e56" xlink:to="loc_country_CA_01a159ac-3da2-799d-925f-2bc262ddf895" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_Assets_01a159ac-3da2-74c5-9661-b8547c742ac1" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_Assets"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_01a159ac-3da2-78c6-a563-fe45912dfb36" xlink:to="loc_us-gaap_Assets_01a159ac-3da2-74c5-9661-b8547c742ac1" xlink:type="arc" order="0"/>
  </link:definitionLink>
</link:linkbase>
