Shawn G. Sheehan was born in Brockton, Massachusetts in 1912. He was the oldest of three children born to Dermond and Annie (Curran) Sheehan, both immigrants from Ireland. Sheehan and his siblings, John and Eileen, grew up in Brockton and attended Brockton High School. He attended the College of the Holy Cross in Worcester, Massachusetts, and graduated in 1933. Sheehan attended St. John's Seminary in Brighton, Massachusetts. Archbishop Richard J. Cushing ordained Sheehan in 1940. Sheehan studied medieval church history at the Catholic University of America, where he wrote a doctoral dissertation entitled "The Christian Reconquest of Spain in the Thirteenth Century: A Study of Principles and Motives." He received his Ph.D. in 1944.

After completing his education, Sheehan was assigned to St. John's Home Missions Seminary in Little Rock, Arkansas in 1945. Sheehan served there until 1948, as part of a "lend-lease" policy for areas in need of priests. After returning to Massachusetts, Sheehan served as assistant pastor at St. Thomas's Parish in Salem and at St. Mary's Parish in Cambridge. He served at St. Mary's from 1948 to 1954. Sheehan returned to St. John's Seminary in 1954 as a member of the faculty. He taught liturgical and church history at the seminary for twelve years until 1966. In 1966, Sheehan received another parish assignment to St. Leo's Parish in Dorchester, Massachusetts. After six years, Sheehan was assigned to St. Elizabeth of Hungary Parish in West Acton. He served at St. Elizabeth's from 1972 until his retirement in 1983. He also served as priest-in-residence at Sacred Heart Parish in Newton Centre.

Besides his service as a parish priest and his work as a professor of church history, Sheehan was also highly involved in the liturgical movement that took shape within the Catholic Church in the 1940s and 1950s. The liturgical movement was led by well-known figures Virgil Michel, Gerald Ellard, Michael Mathis, Martin Hellriegel, Reynold Hillenbrand, and Godfrey Diekmann. The movement sought to bring the laity into greater participation in the liturgy. Sheehan maintained an active presence in the National Liturgical Conference for forty years. He served as secretary from 1947-1948, and as president from 1956-1959. On the regional and local level, Sheehan was a member of the New England and Archdiocesan Sacramental Apostolates. In addition, Sheehan served as editor of the publication Mediator, which advocated renewal of Catholic worship, from 1949-1969.

Sheehan participated in other church movements including ecumenism, and justice and peace. He participated in early Protestant-Catholic dialogues. He also worked with education efforts on behalf of the United States Catholic Bishops' Peace Pastoral. Sheehan remained active in the social issues of justice and peace. In 1965, he participated in the civil rights march in Selma, Alabama. He also worked to bring Rev. Martin Luther King Jr. to speak at St. John's Seminary. During his service at St. Leo's in Dorchester, Sheehan worked with social service agencies, federal antipoverty programs, and The Model Cities Program. Moreover, he served as the first president of Lena Park Development Corporation. While serving as priest-in-residence at Sacred Heart, Sheehan was involved with a Sister City Project between Newton, Massachusetts, and San Juan del Sur, Nicaragua.

Shawn Sheehan died at the age of 78 on October 19, 1990, after a battle with cancer.