[2] McGrath attended various schools in both Latin America and the United States, before graduating from La Salle Military Academy in New York in 1939.

[5] 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.

[2] In August 1942, McGrath entered the novitiate of the Congregation of Holy Cross at Notre Dame and made first vows in September 1943.

[6][7] 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.

In Rome he obtained his doctorate with a thesis qualified as magna cum laude entitled The First Vatican Council's Teaching on the Evolution of the Dogma.

[5] He engaged in pastoral visits and study groups regarding Vatican II, and became vicar capitular of the archdiocese upon Beckmann's death in October 1963.

He was a major contributor to Gaudium et spes, imbuing it with the thought of Joseph Cardijn in regards to the "see, judge, act" method as well as a theology of the signs of the time, and the dignity of the laity by virtue of their baptism.

[5] A priest serving in the diocese, Jesús Héctor Gallego Herrera, was abducted and killed by soldiers after angering landowners and military officials shortly after McGrath became archbishop of Panama.

The archbishop also served as an advocate for a return to democracy and a defender of human rights, especially during and after the 1968 Panamanian coup d'état and the administration of Manuel Noriega.