 Lanfredini was ordained on March 16, 1727 by Pope Benedict XIII.He was appointed successively as Relator of the Sacred Congregation of Good Government, Auditor of the Camerlengo of the Holy Roman Church, Canonist of the Apostolic Penitentiary (1728), Secretary of the Sacred Congregation of the Tridentine Council (1730), Canon of the patriarchal Vatican basilica (1730), Voter of the Supreme Tribunal of the Apostolic Signature (1731), Datary of the Apostolic Penitentiary (1733) and later regent.He served as counselor for Pope Innocent XIII, Pope Benedict XIII, and Pope Clement XII.