French art dealers, brothers. In 1906 they took over the Paris gallery of their father, Alexandre Rosenberg, who had set up in business in 1872, specializing first in Old Masters and later in Impressionist and Post-Impressionist pictures. However, in 1910 they split the business, and for the next few years Léonce was primarily a collector rather than a dealer. He returned to dealing during the First World War, when Kahnweiler's forced absence in neutral Switzerland allowed him to become the main advocate of Cubism by giving contracts to Braque, Gris, and Léger. In 1918 he opened the Galerie de l'Effort Moderne, which for a few years was a powerful force in promoting avant-garde art. Between 1924 and 1927 it issued the Bulletin de l'Effort Moderne (forty issues), which provided a forum not only for his own views, but also, for example, for those of Giorgio de Chirico, Theo van Doesburg, and the critics Maurice Raynal (1884–1954) and Pierre Reverdy (1889–1960). Soon after the war, however, several of the leading artists Rosenberg represented (including Picasso) went over to his brother Paul, and Léonce made himself unpopular in the early 1920s when he acted as an expert for the French government's sale of Kahnweiler's confiscated pre-war stock: this outraged many people in the art world, as it gave Rosenberg ‘a chance to liquidate the stock of his commercial rival at knock-down prices and take advantage of his privileged position to expand his own’ (catalogue of the exhibition ‘The Essential Cubism’, Tate Gallery, London, 1983). During the 1930s his business was badly hit by the Depression.
Paul Rosenberg's gallery was distinctly up-market and he concentrated on promoting established artists rather than fostering new talent. Picasso (1918), Braque (1922), and Léger (1927) all moved from Léonce to Paul, and in 1936 he also became Matisse's dealer. In the period between the world wars he maintained a branch of his gallery in London, and in 1940 he moved his business to New York, where his son Alexandre acted as manager.