Boisgelin was born in Rennes. Achieving remarkable success in his studies at the seminary of Saint-Sulpice and the Sorbonne, the death of his elder brother made him the head of his family, and giving up his birthright, he dedicated his life to the Catholic Church. He rose rapidly through the hierarchy of the church, first made Vicar-General of Pontoise; then in 1765 he was created Bishop of Lavaur; and on 4 November 1770 he was appointed Archbishop of Aix in Provence. Boisgelin also had the honour of delivering the funeral orations of both of the future king Louis XVI's parents, the Dauphin in 1765, and the Dauphine in 1767.