<?xml version="1.0"?>

<ownershipDocument>

    <schemaVersion>X0201</schemaVersion>

    <documentType>4</documentType>

    <periodOfReport>2003-10-01</periodOfReport>


    <notSubjectToSection16>0</notSubjectToSection16>

    <issuer>
        <issuerCik>0001018099</issuerCik>
        <issuerName>MILLENNIUM CHEMICALS INC</issuerName>
        <issuerTradingSymbol>MCH</issuerTradingSymbol>
    </issuer>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001030524</rptOwnerCik>
            <rptOwnerName>MEACHIN DAVID J P</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>351 EAST 84TH STREET, APT. 29E</rptOwnerStreet1>
            <rptOwnerStreet2></rptOwnerStreet2>
            <rptOwnerCity>NEW YORK</rptOwnerCity>
            <rptOwnerState>NY</rptOwnerState>
            <rptOwnerZipCode>10028</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 $0.01 par value/share</value>
            </securityTitle>
            <transactionDate>
                <value>2003-10-01</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>A</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionTimeliness>
                <value></value>
            </transactionTimeliness>
            <transactionAmounts>
                <transactionShares>
                    <value>2094</value>
                </transactionShares>
                <transactionPricePerShare>
                    <value>9.55</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>A</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>6480</value>
                    <footnoteId id="F1"/>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>

    </nonDerivativeTable>

    <footnotes>
        <footnote id="F1">Represents shares issued under the Company's Long Term Stock Incentive Plan in partial payment of annual Director's fee.</footnote>
    </footnotes>


    <ownerSignature>
        <signatureName>David J. P. Meachin</signatureName>
        <signatureDate>2003-10-03</signatureDate>
    </ownerSignature>

</ownershipDocument>

