On July 17, 1883, Pope Leo XIII appointed Riordan to be coadjutor archbishop with the right of succession to Joseph Sadoc Alemany, the Archbishop of San Francisco, California. He was also given the honorary title of titular archbishop of Cabasa. He received his episcopal consecration on the following September 16 from Archbishop Patrick Feehan, with Bishops William George McCloskey and Silas Chatard serving as co-consecrators, at St. James in Chicago.
