 On May 23, 1962, Treinen was appointed the fifth Bishop of Boise in Idaho by Pope John XXIII.He received his episcopal consecration on the following July 25 from Bishop Hilary Baumann Hacker, with Bishops Peter William Bartholome and Lambert Hoch serving as co-consecrators.He attended all four sessions of the Second Vatican Council between 1962 and 1965, and worked to implement the reforms of the Council in the diocese.