Robert Cronin attended Holy Cross College, Worcester, Massachusetts, until 1925, at which time he continued studies at Our Lady of Angels Seminary, in Niagara, New York, receiving his B.A. in 1930. He was ordained a Catholic priest on May 30, 1931. He first served as assistant pastor at the Cathedral of the Immaculate Conception in Albany, New York. In October 1942 he was commissioned a U.S. Navy chaplain, first with the FIRST Marine Division, and subsequently with the 3d Marine Raider Battalion in the Pacific Theater of Operations, where he earned the Silver Star and Purple Heart. He was discharged after the war as a U.S. Navy commander in 1946.