Born in Saint-Sauveur-Lendelin (Manche), after studies of philosophy at the Collège de Navarre, he started his career during the Ancien Régime, making his first appearance as a lawyer in Paris in 1762.

[1] 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).

In the Estates-General and (after he took the Tennis Court Oath) in the National Constituent Assembly, where he sat as deputy for the Third Estate in the bailiwick of Dourdan, he professed Liberalism and proposed various financial laws, without affiliating to any particular faction.

[citation needed] In 1795, Lebrun was elected as a deputy to the French Directory's Council of Ancients and,[1] although a supporter of the House of Bourbon, he voted against prosecutions of Jacobins, and showed himself in favour of national reconciliation.

[1] He opposed Napoleon's restoration of the noblesse and, in 1808, only reluctantly accepted the title of duc de Plaisance (Duke of Piacenza),[1] a rare, nominal, but hereditary duché grand-fief, extinguished in 1926.