Born in Florence, the son of Bartolomeo Corsini, Marquis of Casigliano and his wife Isabella Strozzi, sister of the Duke of Bagnuolo, Corsini had been an aristocratic lawyer and financial manager under preceding pontiffs. He is known for building the new façade of the Basilica di San Giovanni in Laterano, beginning construction of the Trevi Fountain, and the purchase of Cardinal Albani's collection of antiquities for the papal gallery.
Under Pope Benedict XIII (1724–30), the finances of the Papal States had been delivered into the hands of Cardinal Coscia and other members of the curia, who had drained the financial resources of the see. After deliberating for four months, the College of Cardinals selected Cardinal Lorenzo Corsini, 78 years old and with failing eyesight, who had held all the important offices of the Roman Curia. Clement XII was one of the oldest men to be elected Pope. (Joseph Ratzinger was elected Pope Benedict XVI in the April, 2005 conclave three days after turning 78.) As a Corsini, with his mother a Strozzi, the new pope represented a family in the highest level of Florentine society, with a cardinal in every generation for the previous hundred years.
Corsini was a lawyer, with a degree from the University of Pisa, who had practiced law under the able direction of his uncle, Cardinal Neri Corsini. After the death of his uncle and his father, in 1685, Lorenzo, now thirty-three, would have become head of the Corsini. Instead he resigned his right of primogeniture and from Pope Innocent XI (1676–89) 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.
In 1696 Corsini was appointed treasurer-general and governor of the Castel Sant'Angelo. His good fortune increased during the pontificate of Pope Clement XI (1700–21), who employed his talents as a courtier and rewarded him with a cardinal's hat, on May 17, 1706, retaining his services as papal treasurer.
He advanced still further under Pope Benedict XIII, who made him prefect of the judicial tribunal known as the Segnatura di Giustizia. He was successively Cardinal-Priest of San Pietro in Vincoli and Cardinal-Bishop of Frascati.
Though he was blind and compelled to keep to his bed, from which he gave audiences and transacted affairs of state, he surrounded himself with capable officials, many of them his Corsini relatives, but he did little for his family except to purchase and enlarge the palace built in Trastevere for the Riarii, and now known as the Palazzo Corsini (the seat of the Regia Accademia dei Lincei). In 1754, his nephew, Cardinal Neri Corsini, founded there the famous Corsini Library. His first moves as Pope Clement XII were to restore the papal finances. He demanded restitution from the ministers who had abused the confidence of his predecessor. The chief culprit, Cardinal Coscia, was heavily fined and sentenced to ten years' imprisonment. Papal finances were also improved through reviving the public lottery, which had been suppressed by the severe morality of Benedict XIII. Soon it 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, but which he was never able to see.
A competition for the majestic façade of the San Giovanni in Laterano was won by architect Alessandro Galilei. The façade he designed is perhaps more palatial than ecclesiastic, and was finished by 1735. Clement XII erected in that ancient basilica a magnificent chapel dedicated to his 14th century kinsman, St. Andrea Corsini. He restored the Arch of Constantine and built the governmental palace of the Consulta on the Quirinal. He purchased from Cardinal Albani for 60,000 scudi a famous collection of statues, inscriptions, etc., and added it to the gallery of the Capitol. He paved the streets of Rome and the roads leading from the city, and widened the Corso. He began the triumphant Baroque Fontana di Trevi, one of the noted ornaments of Rome. Under his reign a port was built at Ancona, with a highway that gave easy access to the interior. He drained the malarial marshes of the Chiana near Lake Trasimeno.
Politically, however, this was not a successful papacy among the secular powers of Europe. When the attempt of papal forces to take over the ancient independent Republic of San Marino failed, Clement XII disavowed the arbitrary action of his legate, Cardinal Alberoni, in seizing San Marino, and restored its independence. He was also rebuffed in Papal claims over the Duchies of Parma and Piacenza.
In ecclesiastic affairs Clement XII's reign was less propitious. He issued the first papal decree against the Freemasons on April 28, (1738), it was titled In eminenti. He canonized Saint Vincent de Paul and proceeded with vigor against the French Jansenists. He campaigned for the reunion of the Roman and Orthodox churches, received the Patriarch of the Coptic Church and persuaded the Armenian Patriarch to remove the anathema against the Council of Chalcedon and Pope Leo I (440–461). 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. He created the youngest Cardinal ever when on December 19, 1735, he named Luis Antonio Jaime de Borbón y Farnesio, Royal Infant of Spain, age 8, to the Sacred College. He did much to develop schooling in Ottoman Albania as well, as his mother was from Albanian origins and his forefathers were soldiers under the command of Scanderbeg's army.
Pope Clement XII's tomb is in the Capella Corsini of the Basilica of St. John Lateran and was completed by the sculptors Maini and Monaldi. His bust was completed by Filippo della Valle (see terracota bust.).