On March 4, 1966, Gerety was appointed coadjutor bishop of the Diocese of Portland in Maine and titular bishop of "Crepedula" by Pope Paul VI. He received his episcopal consecration on the following June 1 from Archbishop Henry Joseph O'Brien, with Bishops Daniel Joseph Feeney and John Francis Hackett serving as co-consecrators, at the Cathedral of St. Joseph in Hartford. Due to the failing health of Bishop Feeney, Gerety became apostolic administrator of the diocese on February 18, 1967, assuming responsibility for the daily activities of the diocese.