Born from a recently noble Venetian family (the father was from a family of wealthy Genoese merchants of Lombard origin, the Della Torre, who moved to Venice, where they had bought the noble title), by Giovanni Battista and Vittoria Barbarigo, was educated by the Jesuits in Bologna.
He covered several important positions in the Curia. Bishop of Padua from 1743, he became a cardinal in 1737, and then Pope on 6 July 1758: in the same year the Rezzonico family celebrated the marriage of Ludovico Rezzonico with a representative of the powerful Savorgnan family. Clemente XIII was famous for his nepotism.
Despite the meekness and affability of his upright and moderate character, modest to excess (he covered all the classic Vatican statues with the famous fig leaves), and generous with his vast private heritage, his pontificate was disturbed by disputes perpetual about the request for the suppression of the Jesuits who came from the circles of the French Enlightenment. Clemente put the Encyclopédie of D'Alembert and Diderot on the Index of Prohibited Books. Another and unexpected resistance to the Encyclopédie came from the courts of Spain, Portugal and the Kingdom of the Two Sicilies.
In 1758 the reforming minister of Joseph I of Portugal, the Marquis of Pombal, expelled the Jesuits from Portugal, and sent them en masse to Civitavecchia, as a "gift for the Pope". In 1760 the Pombal also sent home the papal nuncio and recalled the Portuguese ambassador. The libel entitled Brief Report represented the Jesuits as creators of a virtually independent kingdom in South America, under their sovereignty, in which the Indians tyrannized, all in the interest of their insatiable ambition and avarice. The theses were false, but the cause of the Jesuits came out damaged.
In France the Parliament of Paris, with its strongly high-bourgeois background and the Jansenist sympathies, gave rise to pressure to obtain the expulsion of the Jesuits in the spring of 1761 and published extracts from Jesuit writings, the Extraits des assertions which, mutilated and taken out of their context, they provided ammunition for the anti-Jesuits. Although a congregation of bishops gathered in Paris in December 1761 recommended not to take action, Louis XV promulgated a royal order that allowed the Society of Jesus to remain in the kingdom, provided that certain essentially liberalizing changes within the institution satisfied the Parliament, with a vicar-general of the French Jesuits who was to be independent of the General in Rome. At the Arrêt of 2 August 1762, with which Parliament suppressed the Jesuits in France, imposing unacceptable conditions, Clement replied with a protest against the invasion of the rights of the Church, and annulled the arrêt. The ministers of Louis XV could not allow such an abrogation of the French law, and Louis finally expelled the Jesuits in November 1764.
Clemente strongly supported the order with a Papal Bull, the Apostolicum Pascendi of January 7, 1765, which rejected the criticism of the Jesuits as slanders and praised the usefulness of the order. This was largely ignored: by 1768 the Jesuits had been expelled from France, the Two Sicilies and the Duchy of Parma. In Spain they seemed to be safe, but Charles III, conscious of the exhausting contests of Bourbon France, decided for prevention. In the night between 2 and 3 April 1767, all the Jesuit houses in Spain were surrounded, the occupants arrested and loaded with their clothes on ships heading for Civitavecchia. The King's letter to Clement XIII promised that his allowance of 100 plates per capita a year would be withdrawn to the whole Order, if any of them had dared to write something in their defense or criticizing the reasons for the expulsion, reasons that he refused to discuss, either now or in the future.
More or less the same fate awaited them in the Bourbon territories of the Duke of Parma and Piacenza, advised by the liberal minister Guillaume du Tillot. In 1768 Clemente issued a strong protest (Monitorium) against the politics of the Parma government. The question of the investiture of Parma came to aggravate Clemente's problems. The Bourbon kings took the parts of their relatives, taking Avignon, Benevento and Pontecorvo, and joining in a peremptory request for the total suppression of the Jesuits (January 1769). Brought to extremes, Clement consented to convene a consistory to consider the moves to be made, but on the eve of the day set for the meeting he died (February 2, 1769), not without suspicions of poisoning (of which, however, there is no evidence).