One of the twins born to the late Matthew and Katherine (Morrison) on May 8, 1936, his twin brother was the late Father Richard J. Butler (2012). The family lived initially in West Roxbury before moving to Belmont. He had three older siblings, Charles who died in 2007; and Matthew and Mary Mullaney who survive him.

Not surprisingly, the twins followed the same paths, graduating from the Boston Public Latin School in 1954, the College of the Holy Cross, Worcester, and St. John Seminary, Brighton.

Richard Cardinal Cushing ordained the twins on Feb. 2, 1962, at Holy Cross Cathedral and gave them their first assignments, with Robert's being to St. Mary of the Nativity, Scituate. From thence forward, their paths diverged, though they both had multiple parish experiences as well as assignments to non-parish ministry.

Following his Scituate appointment, he was named an assistant at St. Philip, Boston; and then at St. Patrick, Stoneham (1967-1969).

He was assigned to graduate studies at Boston College School of Social Work (1969-1971) and was in residence at Sacred Heart, Newton Center. From 1971 to 1986, he was on the staff of the North Shore Office of Catholic Family Counseling and Guidance Services, and in those years, he lived in residence at St. Athanasius, Reading (1971-1985); and Immaculate Conception, Salem (1985-1986).

Cardinal Bernard Law appointed him pastor of St. Mary of the Assumption Parish in Hull on May 13, 1986, and on Jan. 5, 1993, as pastor of St. Joseph Parish, Pepperell.

Very much formed in the teachings of the Second Vatican Council, he was anxious to bring parishioners into more active participation in their faith, not only by embracing the changes in the sacred liturgy but also the side areas such as social involvement and ecumenism.

Newly created Cardinal Seán P. O'Malley granted him senior priest/retirement status on June 1, 2006.

He lived as a senior priest in residence at St. Agatha, Milton, until 2011 when he entered Regina Cleri Residence. At the time of his death on Oct. 21, 2024, he was reported as the longest member of the senior priest community there.

Auxiliary Bishop Peter J. Uglietto was the principal celebrant of Father Butler's funeral Mass celebrated Oct. 29 at St. Joseph Church, Boston. Fellow Regina Cleri resident Father Paul Sughrue was the homilist. Among those joining as concelebrants were Father Robert L. Connors, Braintree; Father Joseph White, pastor, St. Joseph, Boston; Father Brian Clary, Sacred Heart, Roslindale; and several priests from Regina Cleri, including Msgr. William M. Helmick; Father John J. Nichols (both ordination classmates); Father John Mulloy; and Father J. Michael Lawlor.

Following the funeral Mass, Father Butler was buried with his parents and his twin brother at Fairview Cemetery, Hyde Park.