Rezzonico was chosen as Bishop of Padua in 1743 and he received episcopal consecration in Rome by Pope Benedict XIV himself, in the presence of Giuseppe Accoramboni and cardinal Antonio Saverio Gentili as co-consecrators. Rezzonico visited his diocese on frequent occasions and reformed the way that the diocese ran, paying attention to the social needs of the diocese. He was the first to do this in five decades. He later opted to become the Cardinal-Priest of Santa Maria in Aracoeli in 1747 and later still to become the Cardinal-Priest of San Marco in 1755.