Born James Leonard on November 1, 1876 in New York City, he was the son of Bartholomew and Mary Geoghan. James Leonard entered the Passionist novitiate in Pittsburgh, Pennsylvania in the summer of 1894. He professed his vows on February 11, 1896 and received the name Celestine. Theology studies were at Holy Cross Seminary, Dunkirk, New York and then the Angelicum in Rome, Italy. Father Leonard was ordained in Rome on September 21, 1901.
Upon his return to the United States, Father Leonard taught philosophy at Mother of Good Counsel Retreat, Normandy, Missouri. However, increased nervousness prompted Father Leonard to eventually leave the classroom. In 1921 he went to live and minister, until shortly before his death in 1951, as chaplain at St. Vincent Hospital, Normandy. Just prior to his death he also was serving as chaplain for the elderly sisters at Villa St. Louise, Marillac Seminary, Normandy. He was very knowledgeable about local St. Louis Catholic history.