Between his two episcopal appointments, Hayes served as the rector of the Pontifical North American College in Rome from 1935 to 1944.

Hayes studied for the priesthood at the Pontifical North American College and the University of the Congregation of Propagation of the Faith, both in Rome.

[1] Hayes was ordained into the priesthood in Rome by Cardinal Pietro Respighi for the Diocese of Pittsburgh on September 19, 1909.

He then spent seven years as pastor of St. Catherine of Siena Parish in Pittsburgh, doing what he called ‘’real priest’s work.”[1] On June 23, 1933, Pope Pius XI appointed Hayes as the fourth bishop of the Diocese of Helena.

[4] On October 26, 1935, Pope Pius XI appointed Hayes as titular bishop of Hieropolis and rector of the Pontifical North American College.

Hayes also accused the American media of spreading falsehoods about Mussolini and praised the Italian government for its support of the Catholic Church.

[5] With the onset of World War II, Pope Pius XII temporarily closed all the national seminaries, Hayes then returned to Pittsburgh, where he supervised the financial and business matters relating to the college.

[citation needed]Pius XII appointed Hayes as assistant at the pontifical throne on April 30, 1958.

[11] Hayes established the Papal Volunteers of Latin America in the diocese in February 1961 in response to a plea from Pope John XXIII.

[13][14] On his 82nd birthday, September 21, 1966, Hayes submitted his resignation to the Holy See in compliance with the Vatican II decree Christus Dominus.