When did marriage become a sacrament?

Benedicta Ward and G R Evans say in 'The Medieval West', published in A World History of Christianity (edited by Adrian Hastings), page 130, "in the Middle Ages it was never denied that a marriage could quite validly be entered into by two people on their own."

Wikipedia says:

Today many Christian denominations regard marriage as a sacrament, a sacred institution, or a covenant, but this wasn't the case before marriage was officially recognized as a sacrament at the 1184 Council of Verona Before then, no specific ritual was prescribed for celebrating a marriage: "Marriage vows did not have to be exchanged in a church, nor was a priest's presence required. A couple could exchange consent anywhere, anytime."

In early Christian times, the wedding itself was not regarded the sacrament of marriage, because the act of intercourse on the wedding night was the Christian sacrament. Even today, the Catholic Church says a marriage is not made whole (consummated) until the couple has sexual intercourse. Live Science has a convenient online 'History of Marriage' that explains the progress towards marriage as a sacrament that occurs in church:

In 1215, the Catholic Church decreed that partners had to publicly post banns, or notices of an impending marriage in a local parish, to cut down on the frequency of invalid marriages (the Church eliminated that requirement in the 1980s). Still, until the 1500s, the Church accepted a couple's word that they had exchanged marriage vows, with no witnesses or corroborating evidence needed.

The Council of Trent said in Doctrine On The Sacrament Of Matrimony (Session 24, 1563):

Canon I: If any one saith, that matrimony is not truly and properly one of the seven sacraments of the evangelic law, (a sacrament) instituted by Christ the Lord; but that it has been invented by men in the Church; and that it does not confer grace; let him be anathema.

An online essay ('ESSAY 3: A History of Christian Marriage') published as a Report to the 78th General Convention states

page 51: As the centuries progressed into the period we now call the Middle Ages ... A marriage might involve a simple blessing by the priest at the doors of the church, a full nuptial mass within the church, or a blessing of the marriage bed. The consistent holdover from Roman law seems to have been the action that was still most associated with betrothal — namely, the consent to the relationship given by the groom and the agent who gave the bride.

Sacramentaries of the early medieval period resonate with a mishmash of the ideas of Augustine, the sensual sensibilities of Teutonic spirituality, and biblically based understandings of marriage.

By the late medieval period, we see a deepening divide between all things sacred and profane, as well as a fully developed societal and legal authority invested in the officers of the Church. The continuing importance of the betrothal, with its emphasis upon consent and commitment, led to the necessity to make this consent an action done as a part of the marriage rite in the presence of the priest.

As the scholastic church of the late medieval period [generally regarded as the fourteenth and fifteenth centuries] was narrowing its understanding of how Christians were to understand sacrament, marriage (along with its counterpart, ordination) came to be seen as one of the seven sacraments of the Church. [my emphasis]

Martin Luther determined that marriage is not a sacrament.

A formal definition by a council is not always the first instance something was accepted as true (or as a sacrament). Often the council reacted to an attack on an already accepted tenet of faith. Because the attack, the position of the Church had to be officially defined, in order to be formally defended.

There are patristic sources, among whom Augustine, who call Marriage a sacrament, and others who early on describe it in sacramental terms.

"And these are the nuptials of the Lord, so that like that great Sacrament they might become two in one flesh, Christ and the Church. From these nuptials a Christian people is born, when the Spirit of the Lord comes upon that people." Pacian, Sermon on Baptism,6(ante A.D. 392),in JUR,II:144

And

"It is certainly not fecundity only, the fruit of which consists of offspring, nor chastity only, whose bond is fidelity, but also a certain sacramental bond in marriage which is recommended to believers in wedlock. Accordingly it is enjoined by the apostle: 'Husbands, love your wives, even as Christ also loved the Church.' Of this bond the substance undoubtedly is this, that the man and the woman who are joined together in matrimony should remain inseparable as long as they live..." Augustine, On Marriage and Concupiscence,1,10[11](A.D. 420),in NPNF1,V:268

Within the Catholic Church, the International Theological Commission wrote the document 'Propositions on the Doctrine of Christian Marriage'. In it, they write the following, confirming the comments by Matthew on the main question that further clarify that the Church sees this sacrament as going back to Christ:

2.1. Real Symbol and Sacramental Sign

Jesus Christ disclosed in a prophetic way the reality of matrimony as it was intended by God at man’s beginnings (cf. Gen 1:27; 2:24; Mk 10:6, 7-8; Mt 19:4, 5) and restored it through his death and Resurrection. For this reason Christian marriage is lived “in the Lord” (1 Cor 7:39) and is also determined by elements of the saving action performed by Christ.

Already in the Old Testament the matrimonial union was a figure of the Covenant between God and the people of Israel (cf. Hos 2; Jer 3:6-13; Ezek 16 and 23; Is 54), In the New Testament, Christian marriage rises to a new dignity as a representation of the mystery that unites Christ and the Church (cf. Eph 5:21-33). Theological interpretation illuminates this analogy more profoundly: the supreme love and gift of the Lord who shed his blood and the faithful and irrevocable attachment of his Spouse the Church become models and examples for Christian matrimony.

This resemblance is a relationship of real sharing in the Covenant of love between Christ and the Church. From its own standpoint, Christian marriage, as a real symbol and sacramental sign, represents the Church of Christ concretely in the world and, especially under its family aspect, it is called rightly the “domestic Church” (LG 11).

2.2. Sacrament in a Real Sense

In such a way matrimony takes on the likeness of the mystery of the union between Jesus Christ and his Church. This inclusion of Christian marriage in the economy of salvation is enough to justify the title “sacrament” in a broad sense.

But it is also at once the concrete condensation and the real actualization of this primordial sacrament. It follows from this that Christian marriage is in itself a real and true sign of salvation, which confers the grace of God. For this reason the Catholic Church numbers it among the seven sacraments (cf. DS 1327, 1801).

A unique bond exists between the indissolubility of marriage and its sacramentality, that is, a reciprocal, constitutive relationship. Indissolubility makes one s grasp of the sacramental nature of Christian matrimony easier, and from the theological point of view, its sacramental nature constitutes the final grounds, although not the only grounds, for its indissolubility.

Source: http://www.vatican.va/roman_curia/congregations/cfaith/cti_documents/rc_cti_1977_sacramento-matrimonio_en.html