In 1852 Moreau accepts to become the assistant and closest advisor of the first bishop of Saint-Hyacinthe, Most Rev. Prince. He becomes secretary and chancellor of Most Rev. Prince and of his successors. In this role he displays a great capacity for work, order and efficiency. When Most Rev. La Rocque dies on July 15, 1875, Moreau is appointed as his successor and is consecrated fourth bishop of Saint-Hyacinthe on January 16, 1876. He administers the diocese for 25 years. During his term he takes important initiatives such as reopening the episcopal palace in Saint-Hyacinthe, building a cathedral, establishing a chapter, creating a court for matrimonial cases and founding the Sœurs de Saint-Joseph de Saint-Hyacinthe in 1877 and the Sœurs de Sainte-Marthe in 1883. He gives a new impetus to the seminary and improves the intellectual and spiritual training of his priests. Bishop Moreau continues the social work already in progress, in particular keeping a close eye on the development of the Union Saint-Joseph that he had founded. During the 25 years of his administration Moreau founded 13 parishes and 22 institutions, most of which were educational (academies or commercial colleges). He takes part in the debates concerning the universities and Manitoba schools. He dies on May 24, 1901, and is beatified by John-Paul II on May 10, 1987.
