In the company of Étienne Nicolas Méhul, Honoré Langlé, François-Joseph Gossec and Charles Simon Catel, he was limited to teaching elementary principles and solfège.

Unable to get his operas Ossian, ou Les bardes and La mort d'Adam mounted at the Paris Opéra, Le Sueur published a violent pamphlet, Projet d'un plan général de l'instruction musicale en France, attacking the Conservatoire, its methods and its director, and was discharged, 23 September 1802.

[1] Without official appointments, Le Sueur was reduced to poverty when in 1804, Napoleon named him maître de la chapelle at the Tuileries, to replace Giovanni Paisiello.

Now he was able to mount his most famous work, Ossian ou Les bardes, with great success at the Opéra and with the Emperor, who made the composer of his favorite opera a Chevalier de la Légion d'honneur.

From the beginning of 1818, he taught composition at the Conservatoire, where over the years he had for pupils Hector Berlioz, Ambroise Thomas, Charles Gounod, Louis Désiré Besozzi and Antoine François Marmontel.