On the convocation of the États généraux, he was noted for many political writings. Deputy for the Rhône at the Legislative Assembly, he was elected its president several times. He took part in the defence of Lyon against the troops of the National Convention and in 1793 escaped death by fleeing to Switzerland. Lémontey returned in 1795 and was in 1804 made head of the theatrical censorship commission, entering the Académie française in 1819.
He was twice a laureate of the Académie de Marseille for his Éloges praising Peiresc (1785) and those praising Cook (1788). He edited royalist newspapers and was one of the companions at the "Déjeuner de la Fourchette".
Thibaut, ou la Naissance d'un comte de Champagne, poème en 4 chants, sans préface et sans notes, traduit de la langue romance, sur l'original composé en 1250, par Robert de Sorbonne, clerc du diocèse de Rheims (1811)
Essai sur l'établissement monarchique de Louis XIV, et sur les altérations qu'il éprouva pendant la vie de ce Prince (1818) - both praised and controversial on its publication, it was a forerunner of L'Ancien Régime et la Révolution by Tocqueville and underlined the continuity between the institutions of the Ancien Régime and those of the French Revolution. He interpreted this continuity as a result of Louis XIV's absolutism.