Margaret Mary received several private revelations of the Sacred Heart. The visions revealed to her the form of the devotion to the Sacred Heart of Jesus, the chief features being reception of Holy Communion on the first Friday of each month, Eucharistic adoration during a "Holy hour" on Thursdays, and the celebration of the Feast of the Sacred Heart. She stated that in her vision she was instructed to spend an hour every Thursday night to meditate on Jesus' Agony in the Garden of Gethsemane. The Holy Hour practice later became widespread among Catholics. 

Margaret Mary reported that Jesus permitted her to rest her head upon his heart, and then disclosed to her the wonders of his love, telling her that he desired to make them known to all mankind and to diffuse the treasures of his goodness, and that he had chosen her for this work.

Initially discouraged by her peers in her efforts to follow the instruction she had received in her visions, Alacoque was eventually able to convince her superior, Mother de Saumaise, of the authenticity of her visions. She was unable, however, to convince a group of theologians of the validity of her apparitions, nor was she any more successful with many of the members of her own community, and suffered greatly at their hands. She eventually received the support of St. Claude de la Colombière, the community's confessor for a time, who declared that the visions were genuine.

In 1683, opposition in the community ended when Mother Melin was elected Superior and named Margaret Mary her assistant. She later became Novice Mistress, and saw the monastery observe the Feast of the Sacred Heart privately, beginning in 1686. Two years later, a chapel was built at Paray-le-Monial to honor the Sacred Heart.

Alacoque died on 17 October 1690.

After the death of Margaret Mary, devotion to the Sacred Heart was fostered by the Jesuits and the subject of controversies within the Church. The practice was not officially recognized until 75 years later. 

The discussion of Alacoque's own mission and qualities continued for years. All her actions, her revelations, her spiritual maxims, her teachings regarding the devotion to the Sacred Heart, of which she was the chief exponent as well as the apostle, were subjected to the most severe and minute examination, and finally the Sacred Congregation of Rites passed a favorable vote on the heroic virtues of this "servant of God".

In March 1824, Pope Leo XII pronounced her Venerable and on 18 September 1864 Pope Pius IX declared her Blessed. When her tomb was canonically opened in July 1830, two instantaneous cures were recorded to have taken place. Her incorrupt body rests above the side altar in the Chapel of the Apparitions, located at the Visitation Monastery in Paray-le-Monial, and many striking blessings have been claimed by pilgrims attracted there from all parts of the world. 

Alacoque was canonized by Pope Benedict XV in 1920, and in 1929 her liturgical commemoration was included in the General Roman calendar for celebration on 17 October, the day of her death. In the reforms of 1969, the feast day was moved to the prior day, 16 October.

In a 1928 encyclical, Pope Pius XI affirmed the Church's position regarding the credibility of her visions of Jesus Christ by speaking of Jesus as having "manifested Himself" to Saint Margaret Mary and having "promised her that all those who rendered this honor to His Heart would be endowed with an abundance of heavenly graces".