Thomas Gavin was ordained a Catholic Priest in 1911, and was assigned to the Archdiocese of New York. While serving as an Assistant at the Church of the Epiphany in New York City, he entered military service in 1917 and was commissioned a U.S. Army chaplain, serving with the 2d Division in France, where he was cited for gallantry in action. After the war he returned to ministry in New York.