 On 8 November 1570 he was elected Titular Patriarch of Alexandria.He accompanied Cardinal Michele Bonelli in 1571 on his legation to Spain, the Kingdom of Portugal, and the Kingdom of France designed to promote a league against the Ottoman Empire.Riario was consecrated as a bishop in the Sistine Chapel on 24 August 1572 by Cardinal Benedetto Lomellini assisted by Antonio Elio, Titular Patriarch of Jerusalem, and Giovanni Ambrosio Fieschi, Bishop of Savona.