The diocese's previous bishop, Joseph Sadoc Alemany, O.P., had been promoted to archbishop of the newly created Archdiocese of San Francisco.

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.

[citation needed] Amat founded some of the first schools in Los Angeles and asked his fellow Vincentians to open St. Vincent's College (now known as Loyola Marymount University).

He welcomed the Franciscan Brothers of Ireland into his diocese to work in the parochial schools, as well as the Daughters of Charity and the Sisters of the Immaculate Heart of Mary.

Amat formally consecrated Calvary Cemetery on North Broadway (formerly Buena Vista Street) at Bishops Road in 1866.

[3] He dedicated the Gothic Revival brick chapel to Saint Nicholas at the Workman Family Cemetery in the City of Industry.