 As a priest of the Diocese of Bismarck in North Dakota, Treinen was a curate in Dickinson from 1946 to 1950, and secretary to Bishop Vincent James Ryan and his successor, Bishop Lambert Anthony Hoch, from 1950 to 1953.He then served as a curate at the Cathedral of the Holy Spirit (1950–57), chancellor of the diocese (1953–59), and curate at St. Anne's Church in Bismarck (1957–59).From 1959 to 1962, he was pastor of St. Joseph's Church in Mandan.