He was born to a recently ennobled family of Venice, received a Jesuit education in Bologna and became a cardinal in 1737. Previously he had filled various important posts in the Curia and had been bishop of Padua since 1743; during his tenure as bishop of Padua he visited all the parishes in the diocese, the first bishop to do that for 50 years. He became pope on July 6, 1758. In the same year the Rezzonico family were celebrating Ludovico Rezzonico's marriage into the powerful Savorgnan family. The son of the man who bought the unfinished palace on the Grand Canal (now Ca' Rezzonico) and finished its construction, Carlo the pontiff was notorious for his rampant nepotism.
Notwithstanding the meekness and affability of his upright and moderate character, modest to a fault (he had the classical sculptures in the Vatican provided with mass-produced fig leaves) and generous with his extensive private fortune, Clement XIII's pontificate was disturbed by perpetual contentions respecting the pressures to suppress the Jesuits coming from the progressive Enlightenment circles of the philosophes in France. Clement XIII placed the Encyclopédie of D'Alembert and Diderot on the Index, but this index was not as effective as it used to be in the previous century. More unexpected resistance came in the less progressive courts of Spain, the Two Sicilies, and Portugal. In 1758 the reforming minister of Joseph I of Portugal (1750–77), the Marquis of Pombal, expelled the Jesuits from Portugal, and shipped them en masse to Civitavecchia, as a "gift for the Pope." In 1760, Pombal sent home the papal nuncio and recalled the Portuguese ambassador. The pamphlet titled the Brief Relation, which represented the Jesuits as having set up virtually an independent kingdom in South America under their own sovereignty, and of tyrannizing the Native Americans, all in the interest of an insatiable ambition and avarice, whether or not it was completely true, was damaging to the Jesuit cause.
In France, the Parlement de Paris, with its strong upper bourgeois background and Jansenist sympathies, opened the pressure to expel the Jesuits from France in the spring of 1761, and the published excerpts from Jesuit writings, the Extrait des assertions, taken out of context perhaps, certainly provided anti-Jesuit ammunition. Though a congregation of bishops assembled at Paris in December 1761 recommended no action, Louis XV of France (1715–74) promulgated a royal order permitting the Society to remain in the kingdom, with the proviso that certain essentially liberalizing changes in their institution satisfy the Parlement with a French Jesuit vicar-general who should be independent of the general in Rome. To the arrêt of August 2, 1762, by which the Parlement suppressed the Jesuits in France, imposing untenable conditions on any wishing to remain in the country, Clement XIII replied by a protest against the invasion of the Church's rights, and annulled the arrêts. Louis XV's ministers could not permit such an abrogation of French law, and the King finally expelled the Jesuits in November 1764.
Clement XIII warmly espoused the order in a papal bull Apostolicum pascendi, January 7, 1765, which dismissed criticisms of the Jesuits as calumnies and praised the order's usefulness; it was largely ignored: by 1768 the Jesuits had been expelled from France, the Two Sicilies and Parma. In Spain, they appeared to be safe, but Charles III of Spain (1759–88), aware of the drawn-out contentions in Bourbon France, decided on a more peremptory efficiency. During the night of April 2–3, 1767, all the Jesuit houses of Spain were suddenly surrounded, the inhabitants arrested, shipped to the ports in the clothes they were wearing and bundled onto ships for Civitavecchia. The King's letter to Clement XIII promised that his allowance of 100 piastres each year would be withdrawn for the whole order, should any one of them venture at any time to write anything in self-defence or in criticism of the motives for the expulsion, motives that he refused to discuss, then or in the future.
Much the same fate awaited them in the territories of the Bourbon Duke of Parma and Piacenza, advised by the liberal minister Guillaume du Tillot. In 1768 Clement XIII issued a strong protest (monitorium) against the policy of the Parmese government. The question of the investiture of Parma aggravated the Pope's troubles. The Bourbon Kings espoused their relative's quarrel, seized Avignon, Benevento and Pontecorvo, and united in a peremptory demand for the total suppression of the Jesuits (January 1769). Driven to extremes, Clement XIII consented to call a consistory to consider the step, but on the very eve of the day set for its meeting he died (February 2, 1769), not without suspicion of poison, of which, however, there appears to be no conclusive evidence.
From the Annual Register, for 1758: Pope Clement XIII was "the honestest man in the world; a most exemplary ecclesiastic; of the purest morals; devout, steady, learned, diligent..."