After initially beginning his higher pre-medicine studies at St. Bonaventure College in Olean, N.Y., he decided to enter the Roman Catholic religious congregation of the Missionary Oblates of Mary Immaculate in Tewksbury, Mass., in 1933. After finishing his theological studies at Oblate College in Washington D.C., he was ordained to the priesthood by Cardinal Francis Spellman on May 27, 1940.
Fr. Leising received his first assignment to the Arctic Missions of the Mackenzie, Canada, and spent 25 years in those Missions. He began by learning the Chipewayan language. The first ten years he often traveled by dog sled to reach the missions under his care, as well as by boat in the summers.