Cardinal Copello participated in the conclave of 1958, which resulted in the election of Pope John XXIII. He resigned, after twenty-six years of service, as Buenos Aires' archbishop on 25 March 1959, and was appointed Apostolic Chancellor on the same date (remaining in that post until his death). Copello thus became Cardinal-Priest of San Lorenzo in Damaso on 14 December 1959. From 1962 to 1965, he attended the Second Vatican Council, during the course of which he served as a cardinal elector in the 1963 papal conclave that selected Pope Paul VI.
