One of the 10 children of the late John and Mary (Healy) Sullivan, Father John L. Sullivan was born in Boston on May 10, 1948, and raised in the Hub's Roslindale section. He is an alumnus of Roslindale's Sacred Heart Parish School, Boston College High School, and Boston College. His undergraduate specialization is in classical languages.

He entered St. John Seminary and was reunited with his grammar and high school classmate and fellow Roslindale scion, Father Brian Manning, now a senior priest of the archdiocese. They progressed through theological formation, being ordained deacons on May 5, 1973, at Sacred Heart (Italian) Church, Waltham, and to the priesthood at Holy Cross Cathedral by Humberto Cardinal Medeiros on May 18, 1974.

Having served his deacon year at Our Lady of Fatima, Sudbury, he was assigned as a priest to the neighboring town of Natick as an associate at St. Patrick Parish. He was there for two years, but has almost endless stories of the parish's life during the tenure of its pastor, Msgr. Joseph Mahoney, dubbed the "general" because of his regular reference to his service as a military chaplain during the Second World War.

In 1976, he was sent to Rome for graduate studies in Biblical Theology with specialization in the Old Testament. He returned to the United States and the archdiocese in June 1980 and was appointed to the seminary faculty, where he served as professor of Old Testament (1980-2005) and as vice rector (2003-2005). Jovial and marked with a dry and quiet wit, he was much appreciated by the students for his easy demeanor.

In 2005, he returned to parish life, initially not too far from the seminary, as a parochial vicar at St. Columbkille Parish, Brighton. In 2006, he was appointed to St. Gerard Majella Parish, Canton, serving as administrator for a couple months in 2006, and then as pastor until 2012.

In 2012, he was reunited with his Roslindale classmate, Father Brian Manning, when he was named parochial vicar at St. Mary Parish, Franklin, serving there for the next dozen years.

He was granted senior priest/retirement status on June 1, 2024. He will continue to live in residence at the Franklin parish rectory "for as long as Bob Poitras (current Franklin pastor) will have me."