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

    <schemaVersion>X0202</schemaVersion>

    <documentType>4</documentType>

    <periodOfReport>2006-06-29</periodOfReport>

    <notSubjectToSection16>0</notSubjectToSection16>

    <issuer>
        <issuerCik>0000065433</issuerCik>
        <issuerName>MEXICO FUND INC</issuerName>
        <issuerTradingSymbol>MXF</issuerTradingSymbol>
    </issuer>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001332118</rptOwnerCik>
            <rptOwnerName>Gomez Pimienta Jose Luis</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>C/O THE MEXICO FUND, INC.</rptOwnerStreet1>
            <rptOwnerStreet2>1775 I STREET NW</rptOwnerStreet2>
            <rptOwnerCity>WASHINGTON</rptOwnerCity>
            <rptOwnerState>DC</rptOwnerState>
            <rptOwnerZipCode>20006</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>1</isDirector>
            <isOfficer>0</isOfficer>
            <isTenPercentOwner>0</isTenPercentOwner>
            <isOther>0</isOther>
        </reportingOwnerRelationship>
    </reportingOwner>

    <nonDerivativeTable>
        <nonDerivativeTransaction>
            <securityTitle>
                <value>Common Stock</value>
            </securityTitle>
            <transactionDate>
                <value>2006-06-29</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>P</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionTimeliness>
                <value></value>
            </transactionTimeliness>
            <transactionAmounts>
                <transactionShares>
                    <value>100</value>
                </transactionShares>
                <transactionPricePerShare>
                    <value>29.3</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>A</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>18100</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>I</value>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value>See (1) below</value>
                    <footnoteId id="F1"/>
                </natureOfOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>
    </nonDerivativeTable>

    <footnotes>
        <footnote id="F1">This Form 4 is filed pursuant to Section 16(a) of the Securities Exchange Act of 1934, as amended (&quot;Exchange Act&quot;), to report the purchase of 100 shares by Impulsora del Fondo Mexico, S.C. (&quot;Impulsora&quot;), the Issuer's Investment Adviser, which may be attributable to Mr. Gomez Pimienta, as a result of Exchange Act Rule 16a-1(a)(2)(iii). Mr. Gomez Pimienta is the President and Chief Executive Officer of Impulsora and owns almost 24% of its capital. Nevertheless, pursuant to Exchange Act Rule 16a-1(a)(4), Mr. Gomez Pimienta disclaims beneficial ownership of the securities of the Issuer held by Impulsora.</footnote>
    </footnotes>

    <ownerSignature>
        <signatureName>/s/Jose Luis Gomez Pimienta, executed by Dilia M. Caballero pursuant to power of attorney</signatureName>
        <signatureDate>2006-06-30</signatureDate>
    </ownerSignature>
</ownershipDocument>
