Born a German at a time when Alsace was not French, Léon Lehmann decided to live in France. After attending secondary school in Belfort. In 1891 to prepare for entry to the École des Beaux-Arts, but withdrew to the Trappe-d'Acey in 1898 and remained there for two years to recover his health. He was befriended by the family of Georges Rouault, whom he had met in Gustave Moreau's studio, and he lived with them for 14 years. He also knew Matisse, Dufy, Asselin and Marquet. World War I tested his health once more, but he was back to painting at the end of it. He died having just completed the decorations of the chapel of Voirons.
He exhibited in Paris at the Salons des Artistes Français, des Indépendants and d'Automne, where a room was devoted to his works in 1936. Retrospectives were held in 1958 at the Musée des Beaux-Arts, Besançon, and in 1963 in a Parisian gallery.