Pegis began teaching in 1931 on the philosophy faculty of his alma mater, the University of Marquette, as an instructor and later as an assistant professor. In 1937 he left Marquette to take a teaching position at
Fordham University, and he returned to the University of Toronto in 1944, where he took posts as a professor of philosophy in the Graduate Department of Philosophy, and as a professor of the history of philosophy in the Pontifical Institute of Mediaeval Studies In 1946 Pegis was elected the First
Fellow of the institute, and served as its president from 1946 to 1954. Pegis was also elected President of the
American Catholic Philosophical Association in 1946, and in 1950 he was elected a
Fellow of the Royal Society of Canada. Pegis left the Pontifical Institute of Mediaeval Studies in 1954 to assume the editorial directorship of
Doubleday's Catholic textbook division. In 1961 he returned to Toronto and resumed full time teaching, both at the Institute and at the University of Toronto. Despite becoming
emeritus in 1971, he was asked, on account of his popularity, to continue his graduate lectures, which he did until his retirement in 1974. During his retirement he worked to develop the Center of Thomistic Studies at the
University of St. Thomas in Houston, where he lectured on the philosophy of
Thomas Aquinas,
Edmund Husserl, and
Martin Heidegger. He continued to give lectures on philosophy until just a few days before his death. He died on May 13, 1978, in
Wellesley Hospital, Toronto. == Philosophy ==