Amat was consecrated as a bishop in Rome on March 12, 1854 by Cardinal Fransoni, the Prefect of the Sacred Congregation for the Propagation of the Faith. Recognizing the growth of Los Angeles and the decline of Monterey, he petitioned the Holy See to move the see to Los Angeles and to be known as Bishop of Los Angeles. Amat arrived in the pueblo of Los Angeles in 1855. On July 7, 1859, the diocese was renamed the Diocese of Monterey-Los Angeles. The Co-Cathedral of Saint Vibiana was founded in Los Angeles and consecrated during the episcopacy of Amat, and he himself brought back from Rome the relics of its patron saint, which were interred in a sarcophagus above the cathedral's main altar.