In 1831, at the age of 17, he travelled to Paris to study art under Jean Auguste Dominique Ingres, becoming one of his most accomplished pupils and a close associate for many years.

In Rome Lehmann befriended Franz Liszt and his lover, the author Marie d'Agoult, corresponding with them for many years and painting portraits of them.

He was awarded many commissions for large-scale public works, such as at the Hôtel de Ville, the Church of Ste-Clothilde, the Palais du Luxembourg, the Palais de Justice, and the Chapel of the Jeunes Aveugles in the Church of Saint-Merri on Rue Saint-Martin.

In 1846 Lehmann received the Légion d'honneur and in 1847 became a French citizen, opening his studio in that same year.

He painted numerous portraits of celebrated contemporaries, including Edmond About, Marie d'Agoult, Princess Christina Belgiojoso, Chopin, Victor Cousin, Charles Gounod, Ingres, Liszt, and Stendhal.