On February 10, 1953, Casey was appointed Auxiliary Bishop of Rochester and Titular Bishop of Cea by Pope Pius XII. He received his episcopal consecration on the following May 5 from Cardinal Francis Spellman, with Bishops Walter Andrew Foery and Alexander M. Zaleski serving as co-consecrators. Following the death of Bishop James Johnston Navagh, he was named Bishop of Paterson, New Jersey, on March 4, 1966. After eleven years as bishop, he resigned on June 13, 1977. Casey died just two days later, aged 71.