 Rezzonico was born on 25 Apr 1724 in Venice, Italy.On 21 Mar 1773, he was consecrated bishop by Giovanni Francesco Albani, Cardinal-Bishop of Porto e Santa Rufina, with Giuseppe Maria Contesini, Titular Archbishop of "Athenae", and Orazio Mattei, Titular Archbishop of "Colossae", serving as co-consecrators.He served as Vice-Chancellor of the Holy Roman Church (1758–1763), Camerlengo of the Holy Roman Church (1763–1799) and Secretary of the Roman Inquisition (1777–1799).