Robert Hearn began preparing for the priesthood at age 14. He graduated from the Church of our Lady of Perpetual Help Parochial School and High School in Roxbury, Massachusetts. He then entered the Redemptorist Preparatory College in Pennsylvania. During his summer vacations he played semiprofessional baseball. In 1924 he began a year of introspection and prayer at a Redemptorist novitiate in Ilchester Maryland, and from 1925 to 1930 he continued studies at Esopus, New York. On June 29, 1930, he was ordained a Redemptorist (Catholic) priest and said his first Mass at the Mission Church in his old neighborhood. He worked as a priest in Pennsylvania and as a missionary in Brazil before becoming a civilian chaplain in 1940. The following year he applied for an Army commission in order to minister to front-line troops. After jungle training in Panama, he joined the 158th Regimental Combat Team in combat in New Guinea, and in 1944 transferred to the 1st Infantry Regiment of the 6th Infantry Division, serving in the New Guinea and Philippine Islands campaigns. He stayed in service after the war and was assigned to the 24th Infantry Division, entering combat again in Korea in 1950, serving there until 1951. In 1957 he retired as a U.S. Army colonel.