He thus became known for building the new façade of the Basilica of Saint John Lateran, beginning construction of the Trevi Fountain,[2] and the purchase of Cardinal Alessandro Albani's collection of antiquities for the papal gallery.

Instead, he renounced his right of primogeniture and from Pope Innocent XI (1676–1689) he purchased, according to the custom of the time, for 30,000 scudi, a position of prelatial rank and devoted his wealth and leisure to the enlargement of the library bequeathed to him by his uncle.

[4] In 1690 he was made titular Archbishop of Nicomedia and chosen nuncio to Vienna, receiving a dispensation from Pope Alexander VIII since he had not yet been ordained a priest.

His good fortune increased during the pontificate of Pope Clement XI (1700–1721),[3] who employed his talents as a courtier and named him Cardinal-Priest of Santa Susanna on 17 May 1706, retaining his services as papal treasurer.

[3] Under Benedict XIII, the finances of the Papal States had been delivered into the hands of Cardinal Niccolò Coscia and other members of the curia, who had drained the financial resources of the see.

Benedict died in 1730, and in the conclave that followed his death, after deliberating for four months, the College of Cardinals selected Corsini, 78 years old and with failing eyesight, who had held all the important offices of the Roman Curia.

As a Corsini, with his mother a Strozzi, the new pope represented a family at the highest level of Florentine society, with a cardinal in every generation for the previous hundred years.

Soon money poured into Clement XII's treasury, an annual sum amounting to nearly a half million scudi, enabling him to undertake the extensive building programs for which he is chiefly remembered,[3] but which he was never able to see.

[3] In August 1730 he gave permission for Victor Amadeus II of Savoy to carry out a morganatic marriage to Anna Canalis di Cumiana.

He dispatched Joseph Simeon Assemani to the East for the twofold purpose of continuing his search for manuscripts and presiding as legate over a national council of Maronites.

[3] He created the youngest Cardinal ever when on 19 December 1735, he named Luis Antonio Jaime de Borbón y Farnesio, Royal Infante of Spain, age 8, to the Sacred College.

[3] Pope Clement XII's tomb is in the Capella Corsini of the Basilica of St. John Lateran and was completed by the sculptors Giovanni Battista Maini and Carlo Monaldi.