As a cardinal, he served as co-president of the Council of Trent in 1545, with cardinals Cervini (later Pope Marcellus II) and Pole. Elected pope on Feb. 7, 1550, he realized that a reform of the church was urgent, and he appointed a commission that recommended resumption of the Council of Trent, which Julius reopened on May 1, 1551. Unfortunately, military and political problems led to its second suspension in 1552. He attempted to stop cardinals from receiving too many benefices and to restore monastic discipline.

His interest in the Society of Jesus (Jesuit) (a religious order founded by Ignatius of Loyola) influenced him to establish the Collegium Germanicum (1552) to train German priests in Rome under the Jesuits. His patronage of Renaissance thought led him to reform the Roman University, to build the Church of St. Andrew in Rome, and to appoint Palestrina choirmaster of St. Peter's, with Michelangelo as the church's principal architect.