During the early 1760s, Lebrun became a disciple of Montesquieu and an admirer of the British Constitution, travelling through Southern Netherlands, the Dutch Republic, and finally to the Kingdom of Great Britain (where he witnessed the debates in the London Parliament).
He became one of Chancellor René Nicolas de Maupéou's chief advisers, taking part in his struggle against the parlements and sharing his downfall in 1774. Lebrun then devoted himself to literature, translating Torquato Tasso's Jerusalem Delivered (1774) and the Iliad (1776). He retreated from public life to his property in Grillon, attempting to live a life as envisaged by the philosophe Jean-Jacques Rousseau. During the cabinet of Jacques Necker, he was consulted on several occasions, but never appointed to high office.
After the voting of the 1791 Constitution, ineligible to the Legislative Assembly (as all former members of the Constituent Assembly), he became instead president of the directory of Seine-et-Oise département.
Lebrun retired from this position on August 7, 1792, and again retired to Dourdan. Three days later, the storming of the Tuileries Palace signalled the move towards the establishment of the French Republic - with the creation of the National Convention. Lebrun further arose the indignation of republicans when he accepted to represent Dourdan in the electoral college of Seine-et-Oise nominating the deputies to the Convention.
In 1795, Lebrun was elected as a deputy to the French Directory's Council of Ancients and, although a supporter of the House of Bourbon, he voted against prosecutions of Jacobins, and showed himself in favor of national reconciliation.
He opposed Napoleon's restoration of the noblesse, and, in 1808, only reluctantly accepted the title duc de Plaisance (Piacenza), a rare, nominal, but hereditary duché grand-fief, extinguished in 1927. From 1811 to 1813 he served as governor-general of Holland, reorganizing its départements - Zuyderzée and Bouches-de-la-Meuse.
Although to a certain extent opposed to the autocracy of the Emperor, he was not in favor of his deposition, although he accepted the fait accompli of the Bourbon Restoration in April 1814. Louis XVIII made him a Peer of France, but during the subsequent Hundred Days he accepted from Napoleon the post of grand maître de l'Université. As a consequence, he was suspended from peerage when the Bourbons returned again in 1815, but was recalled in 1819. He died five years later in Sainte-Mesme (then in Seine-et-Oise, now in Yvelines).
| Preceded by:|
College of 3 Provisional Consuls
| Head of State of France|
(Third Consul along with:)
(Dec. 12 1799 - May 18, 1804)
| Succeeded by:|
(Emperor of the French)