<?xml version="1.0"?>
<ownershipDocument>

    <schemaVersion>X0508</schemaVersion>

    <documentType>4</documentType>

    <periodOfReport>2025-08-22</periodOfReport>

    <issuer>
        <issuerCik>0000036966</issuerCik>
        <issuerName>FIRST HORIZON CORP</issuerName>
        <issuerTradingSymbol>FHN</issuerTradingSymbol>
    </issuer>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001186181</rptOwnerCik>
            <rptOwnerName>POPWELL DAVID T</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>165 MADISON AVENUE</rptOwnerStreet1>
            <rptOwnerStreet2></rptOwnerStreet2>
            <rptOwnerCity>MEMPHIS</rptOwnerCity>
            <rptOwnerState>TN</rptOwnerState>
            <rptOwnerZipCode>38103</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isOfficer>1</isOfficer>
            <officerTitle>SEVP Sr Strategic Executive</officerTitle>
        </reportingOwnerRelationship>
    </reportingOwner>

    <aff10b5One>0</aff10b5One>

    <nonDerivativeTable>
        <nonDerivativeTransaction>
            <securityTitle>
                <value>Common Stock</value>
            </securityTitle>
            <transactionDate>
                <value>2025-08-22</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>S</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionTimeliness></transactionTimeliness>
            <transactionAmounts>
                <transactionShares>
                    <value>100000</value>
                </transactionShares>
                <transactionPricePerShare>
                    <value>22.4952</value>
                    <footnoteId id="F1"/>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>D</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>462581</value>
                    <footnoteId id="F2"/>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>
        <nonDerivativeHolding>
            <securityTitle>
                <value>Common Stock</value>
            </securityTitle>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>3159</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>I</value>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value>401(k)</value>
                </natureOfOwnership>
            </ownershipNature>
        </nonDerivativeHolding>
    </nonDerivativeTable>

    <footnotes>
        <footnote id="F1">The price reported in Column 4 is a weighted average price.  These shares were sold in multiple transactions at prices ranging from $22.435 to $22.545, inclusive.  The reporting person undertakes to provide to First Horizon Corporation, any security holder of First Horizon Corporation, or the staff of the Securities and Exchange Commission, upon request, full information regarding the number of shares sold at each separate price within the ranges referenced in this footnote.</footnote>
        <footnote id="F2">The amount of shares was increased by 12,570 to correct a computational error that first appeared in an Amendment to Form 4 filed on May 24, 2023 (the &quot;Amendment to Form 4&quot;).  The Amendment to Form 4 reported an administrative error in computing the number of shares withheld to pay withholding taxes, but the Column 5 amount on the Amendment to Form 4 was computed without first adding back the incorrect number of withheld shares (12,570) reported on the original Form 4, filed on May 15, 2023.  As a result, the Amendment to Form 4, as filed on May 24, 2023, and all subsequent Form 4s filed through July 23, 2025, understated the Column 5 amount by 12,570 shares.</footnote>
    </footnotes>

    <ownerSignature>
        <signatureName>/s/ Peter V. Letsou, attorney-in-fact</signatureName>
        <signatureDate>2025-08-26</signatureDate>
    </ownerSignature>
</ownershipDocument>
