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

<!--XBRL Document Created with the Workiva Platform-->
<!--Copyright 2026 Workiva-->
<!--r:01a06c80-e570-7e08-a672-e9a7691ad4e0,g:e5e3cbfb-e0fb-448c-b6ab-7d708243a7e3-->
<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://www.markelcorp.com/role/Cover" xlink:type="simple" xlink:href="mkl-20260903.xsd#Cover"/>
  <link:definitionLink xlink:role="http://www.markelcorp.com/role/Cover" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_dei_DocumentInformationLineItems_01a06c80-e570-7dce-8ba4-28d85850fdb0" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_DocumentInformationLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_DocumentInformationTable_01a06c80-e570-77cb-9cde-816b1c46a5da" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_DocumentInformationTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_dei_DocumentInformationLineItems_01a06c80-e570-7dce-8ba4-28d85850fdb0" xlink:to="loc_dei_DocumentInformationTable_01a06c80-e570-77cb-9cde-816b1c46a5da" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_DocumentInformationDocumentAxis_01a06c80-e570-7a89-8331-67cafb88d43f" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_DocumentInformationDocumentAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_dei_DocumentInformationTable_01a06c80-e570-77cb-9cde-816b1c46a5da" xlink:to="loc_dei_DocumentInformationDocumentAxis_01a06c80-e570-7a89-8331-67cafb88d43f" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_DocumentDomain_01a06c80-e570-7a89-8331-67cafb88d43f_default" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_DocumentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_dei_DocumentInformationDocumentAxis_01a06c80-e570-7a89-8331-67cafb88d43f" xlink:to="loc_dei_DocumentDomain_01a06c80-e570-7a89-8331-67cafb88d43f_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_DocumentDomain_01a06c80-e570-7ff3-b9cd-402acfc7b4d8" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_DocumentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_dei_DocumentInformationDocumentAxis_01a06c80-e570-7a89-8331-67cafb88d43f" xlink:to="loc_dei_DocumentDomain_01a06c80-e570-7ff3-b9cd-402acfc7b4d8" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_DocumentType_01a06c80-e570-7de0-9c69-bc7164dd3869" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_DocumentType"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_01a06c80-e570-7dce-8ba4-28d85850fdb0" xlink:to="loc_dei_DocumentType_01a06c80-e570-7de0-9c69-bc7164dd3869" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_DocumentPeriodEndDate_01a06c80-e570-70d1-9f22-cf3f02bc7b32" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_DocumentPeriodEndDate"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_01a06c80-e570-7dce-8ba4-28d85850fdb0" xlink:to="loc_dei_DocumentPeriodEndDate_01a06c80-e570-70d1-9f22-cf3f02bc7b32" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityRegistrantName_01a06c80-e570-7706-b6e6-808c3107099a" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_EntityRegistrantName"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_01a06c80-e570-7dce-8ba4-28d85850fdb0" xlink:to="loc_dei_EntityRegistrantName_01a06c80-e570-7706-b6e6-808c3107099a" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityIncorporationStateCountryCode_01a06c80-e570-7074-b0e7-6a48d0a1c86c" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_EntityIncorporationStateCountryCode"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_01a06c80-e570-7dce-8ba4-28d85850fdb0" xlink:to="loc_dei_EntityIncorporationStateCountryCode_01a06c80-e570-7074-b0e7-6a48d0a1c86c" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityFileNumber_01a06c80-e570-7c3e-b576-2281a8d33f0b" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_EntityFileNumber"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_01a06c80-e570-7dce-8ba4-28d85850fdb0" xlink:to="loc_dei_EntityFileNumber_01a06c80-e570-7c3e-b576-2281a8d33f0b" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityTaxIdentificationNumber_01a06c80-e570-7d82-a5a8-b1832797de59" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_EntityTaxIdentificationNumber"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_01a06c80-e570-7dce-8ba4-28d85850fdb0" xlink:to="loc_dei_EntityTaxIdentificationNumber_01a06c80-e570-7d82-a5a8-b1832797de59" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityAddressAddressLine1_01a06c80-e570-775d-b942-7e353bc9680c" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_EntityAddressAddressLine1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_01a06c80-e570-7dce-8ba4-28d85850fdb0" xlink:to="loc_dei_EntityAddressAddressLine1_01a06c80-e570-775d-b942-7e353bc9680c" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityAddressCityOrTown_01a06c80-e570-7a95-b0da-ce965eecbeb0" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_EntityAddressCityOrTown"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_01a06c80-e570-7dce-8ba4-28d85850fdb0" xlink:to="loc_dei_EntityAddressCityOrTown_01a06c80-e570-7a95-b0da-ce965eecbeb0" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityAddressStateOrProvince_01a06c80-e570-7cd1-9395-fa9949962486" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_EntityAddressStateOrProvince"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_01a06c80-e570-7dce-8ba4-28d85850fdb0" xlink:to="loc_dei_EntityAddressStateOrProvince_01a06c80-e570-7cd1-9395-fa9949962486" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityAddressPostalZipCode_01a06c80-e570-7d4e-a6ea-3f7effc927de" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_EntityAddressPostalZipCode"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_01a06c80-e570-7dce-8ba4-28d85850fdb0" xlink:to="loc_dei_EntityAddressPostalZipCode_01a06c80-e570-7d4e-a6ea-3f7effc927de" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_CityAreaCode_01a06c80-e570-73b7-9764-359db3798221" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_CityAreaCode"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_01a06c80-e570-7dce-8ba4-28d85850fdb0" xlink:to="loc_dei_CityAreaCode_01a06c80-e570-73b7-9764-359db3798221" xlink:type="arc" order="10"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_LocalPhoneNumber_01a06c80-e570-711f-ac48-c1860032b96d" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_LocalPhoneNumber"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_01a06c80-e570-7dce-8ba4-28d85850fdb0" xlink:to="loc_dei_LocalPhoneNumber_01a06c80-e570-711f-ac48-c1860032b96d" xlink:type="arc" order="11"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_WrittenCommunications_01a06c80-e570-7b82-ae50-8f34defc42b6" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_WrittenCommunications"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_01a06c80-e570-7dce-8ba4-28d85850fdb0" xlink:to="loc_dei_WrittenCommunications_01a06c80-e570-7b82-ae50-8f34defc42b6" xlink:type="arc" order="12"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_SolicitingMaterial_01a06c80-e570-71d1-9ca4-976a0697e95f" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_SolicitingMaterial"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_01a06c80-e570-7dce-8ba4-28d85850fdb0" xlink:to="loc_dei_SolicitingMaterial_01a06c80-e570-71d1-9ca4-976a0697e95f" xlink:type="arc" order="13"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_PreCommencementTenderOffer_01a06c80-e570-79f3-8378-a6bb50de371b" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_PreCommencementTenderOffer"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_01a06c80-e570-7dce-8ba4-28d85850fdb0" xlink:to="loc_dei_PreCommencementTenderOffer_01a06c80-e570-79f3-8378-a6bb50de371b" xlink:type="arc" order="14"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_PreCommencementIssuerTenderOffer_01a06c80-e570-751a-9a09-33b48cae0ff8" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_PreCommencementIssuerTenderOffer"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_01a06c80-e570-7dce-8ba4-28d85850fdb0" xlink:to="loc_dei_PreCommencementIssuerTenderOffer_01a06c80-e570-751a-9a09-33b48cae0ff8" xlink:type="arc" order="15"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_Security12bTitle_01a06c80-e570-7a5c-a2aa-78f8b26bdfde" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_Security12bTitle"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_01a06c80-e570-7dce-8ba4-28d85850fdb0" xlink:to="loc_dei_Security12bTitle_01a06c80-e570-7a5c-a2aa-78f8b26bdfde" xlink:type="arc" order="16"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_TradingSymbol_01a06c80-e570-76df-a037-4a9cc6b4b37f" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_TradingSymbol"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_01a06c80-e570-7dce-8ba4-28d85850fdb0" xlink:to="loc_dei_TradingSymbol_01a06c80-e570-76df-a037-4a9cc6b4b37f" xlink:type="arc" order="17"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_SecurityExchangeName_01a06c80-e570-7872-8c6c-3fd8e8296c1e" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_SecurityExchangeName"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_01a06c80-e570-7dce-8ba4-28d85850fdb0" xlink:to="loc_dei_SecurityExchangeName_01a06c80-e570-7872-8c6c-3fd8e8296c1e" xlink:type="arc" order="18"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityEmergingGrowthCompany_01a06c80-e570-7fe7-951a-5f2e02436469" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_EntityEmergingGrowthCompany"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_01a06c80-e570-7dce-8ba4-28d85850fdb0" xlink:to="loc_dei_EntityEmergingGrowthCompany_01a06c80-e570-7fe7-951a-5f2e02436469" xlink:type="arc" order="19"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_AmendmentFlag_01a06c80-e570-702b-86c8-79c9eb618568" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_AmendmentFlag"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_01a06c80-e570-7dce-8ba4-28d85850fdb0" xlink:to="loc_dei_AmendmentFlag_01a06c80-e570-702b-86c8-79c9eb618568" xlink:type="arc" order="20"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityCentralIndexKey_01a06c80-e570-78d1-9971-96d91c1ee258" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_EntityCentralIndexKey"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_01a06c80-e570-7dce-8ba4-28d85850fdb0" xlink:to="loc_dei_EntityCentralIndexKey_01a06c80-e570-78d1-9971-96d91c1ee258" xlink:type="arc" order="21"/>
  </link:definitionLink>
</link:linkbase>
