McGrath was born in
Ancón, Panama, on 10 February 1924, to John Thomas McGrath of
Trenton, New Jersey, and Louise Renauld of
Cartago, Costa Rica. His father came to Panama in 1912 to work on the
Panama Canal, eventually serving as the captain of a dredge boat. Louise and John had four sons—John, Robert, Eugene, and Mark—before an accident killed John in 1928. McGrath attended various schools in both Latin America and the United States, before graduating from
La Salle Military Academy in New York in 1939. He briefly studied at the
Pontifical Catholic University of Chile from 1939 to 1940 before enrolling at the
University of Notre Dame from 1940 to 1942. During his time there, he encountered the thought of
Catholic Action and the
Young Christian Workers through the influence of Fr. Louis Putz CSC, who in turn had been influenced by
Joseph Cardijn. He also became interested in current events in Latin America, attending international conferences regarding the region and speaking on the topic frequently after becoming a member of the Notre Dame Speaker's Bureau. Upon completion of these studies, he was ordained to the priesthood on 11 June 1949 in the
Cathedral Basilica of St. Mary by
Bishop José María Preciado y Nieva. == Priesthood ==