On March 14, 1944, he was appointed Titular Bishop of Liviade and Auxiliary Bishop of Genoa. Siri received his episcopal consecration on the following May 7 from Pietro Cardinal Boetto, SJ, at the cathedral of S. Lorenzo. He was promoted to Archbishop on May 14, 1946, and created Cardinal Priest of Santa Maria della Vittoria by Pope Pius XII in the consistory of January 12, 1953; he thereby became the youngest member of the College of Cardinals at that time.
Siri was noted for his staunchly conservative views. At the Second Vatican Council (1962-1965), he sat on its Board of Presidency and, alongside Archbishop Marcel Lefebvre, Alfredo Cardinal Ottaviani and Thomas Cardinal Cooray he was part of the association of traditional Council fathers named Coetus Internationalis Patrum. He was also opposed to collegiality and innovation.
Pope John XXIII named Siri the first President of the Italian Episcopal Conference on October 12, 1959, and remained in that post until 1965. Siri, who had voted in the conclaves of 1958 and 1963, was also one of the cardinal electors in the August and October 1978 conclaves. He was a strong candidate for the papacy, or papabile, in all four conclaves, in which his support lay mostly with Curialists and other conservative cardinals. Media reports suggested that Siri in fact topped the first count of votes in the August 1978 conclave but ultimately was beaten by Albino Cardinal Luciani, who became Pope John Paul I. Following John Paul I's death, Siri was the leading conservative candidate in opposition to Giovanni Cardinal Benelli, the Archbishop of Florence and leading liberal candidate. Vaticanologists suggested that the eventual winner, Karol Cardinal Wojtyła, who became Pope John Paul II, was chosen as a compromise candidate between the two. Shortly afterwards, Siri implied that he disapproved of Wojtyła's election.
In a biography of the Cardinal, Nicla Buonasorte reports that, while Siri was a friend of Archbishop Lefebvre, he disapproved of the latter's "schism from Rome" (words of Buonasorte) in 1988, appealing to him ("on his knees", the Cardinal wrote) not to take that step of unlawfully consecrating bishops. In the end, Siri - again, according to Buonasorte - admitted that there was no alternative to excommunicating the dissident French bishop. Buonasorte however fails to deal with the question that the excommunication pronounced after the episcopal consecrations at Ecône was said to have been latae sententiae, not ferendae sententiae, and that the problems surrounding the decree Ecclesia Dei are not addressed by Siri.
The biographer comments: "In all probability, it is due to Siri that Lefebvre had no significant following in Italy" (Siri, il cardinale dell'Ostpolitik segreta in Corriere della Sera, 13 December 2006).
The same book recalls how Siri seems to have turned a blind eye to the assistance given by one or two of his clergy to members of the German National Socialist Party, including Adolf Eichmann, fleeing to South America after the Second World War. It stresses, however, that this was out of compassion for people in difficulties, and quite unconnected with his well-known conservative views, in spite of which he conducted a personal "Ostpolitik" of contacts with the Soviet embassy in favour of the Church in Eastern Europe, without informing Pius XII of these contacts. Cardinal Siri during the war had supported Christian-Democrat Italian resistance financially and morally. He also aided and sheltered some of his priests who tried to help rescue threatened Jews to safe and neutral Spain.
Although claimed by one sedevacantist group that he had actually been elected to the papacy in 1958 and 1963, only to be displaced by Angelo Roncalli (Pope John XXIII) and then Giovanni Battista Montini (Pope Paul VI), Siri entirely submitted to the authority of the official popes and remained in full communion with the Church, refusing to support any sedevacantist organization. One small sedevacantist group, centered in Houston, Texas still claims him to have been the actual pope, despite Siri's own silence as to this claim. This small group, known as "Sirianists", have yet to offer any reasonable explanation for the fact that Siri failed to support the Traditionalist Catholic movement, that he recognized John XXIII, Paul VI, John Paul I and John Paul II as legitimate popes and made public acts of obedience to all of them at the Vatican, that despite his conservatism used the Mass according to the reformed 1970 Roman Missal and the other revised sacraments, and that he signed all of the documents of the Second Vatican Council. The Cardinal even somewhat defended the Council, as long as it was interpreted in the light of Tradition, though he did remark that, "If the Church were not divine, this Council would have buried it. To explain Siri's silence as to what happened in the conclaves, supporters of the Siri Thesis have suggested that Siri was silenced by the conspirators' use of the Seal of the Confessional. This method of silencing prelates has been suggested by other literature.