He attended Boston College, where he earned a Bachelor of Arts degree in 1941.
He served as Bishop of Springfield in Massachusetts from 1977 to 1991. He then began his studies for the priesthood at Saint John"s Seminary in Brighton. He was ordained a priest for the Archdiocese of Boston on June 29, 1945.
Maguire"s first assignment was as a curate at Saint Joseph"s Church in Lynn.
He then served at Saint Anne"s Church in Readville, Blessed Sacrament Church in Jamaica Plain, and Saint Mary of the Hills Church in Milton. From 1956 to 1959, he served as a chaplain in the United States Army Reserve and the Massachusetts National Guard.
He served as secretary to Cardinal Richard Cushing and his successor, Cardinal Humberto Sousa Medeiros, between 1962 and 1971. He was pastor of Saint John the Baptist Church in Quincy from 1971 to 1972.
On December 1, 1971 Maguire was appointed auxiliary bishop of Boston and titular bishop of Mactaris by Pope Paul VI. He received his episcopal consecration on February 2, 1972 from Cardinal Medeiros, with bishops Jeremiah Francis Minihan and Thomas Joseph Riley serving as co-consecrators.
He was named coadjutor bishop of the Diocese of Springfield on April 13, 1976. Upon the resignation of Bishop Christopher Joseph Weldon, Maguire succeeded him as the fifth Bishop of Springfield on October 15, 1977. He remained bishop of the diocese until his retirement, which took place on December 27, 1991.