He was made archbishop of Siena and cardinal deacon in 1460 by his uncle, Pope Pius II (formerly Cardinal Aneas Silvius Piccolomini), who permitted him to assume the name and arms of the Piccolomini. He was employed by subsequent popes in several important legations, as by Paul II at the Diet of Regensburg (1471) and by Innocent VIII to restore ecclesiastical authority in the Italian compartimento of Umbria.

He opposed the flagrant nepotism of Pope Alexander VI, after whose death Cesare Borgia, Alexander's son, seized the Vatican with his troops in an attempt to control the conclave. Protected by the Romans, the cardinals gathered in the Church of the Minerva and on Sept. 22, 1503, elected Francesco, who took the name of Pius. He was consecrated on the following October 1 but died just days after his coronation. In honour of Pius II he founded the Piccolomini Library adjoining the Siena Cathedral.