His gallery quickly gained prominence, for he not only exhibited work by the Abstract Expressionists, but also European artists such as Pierre Bonnard, Paul Klee, Joan Miró, and Piet Mondrian.
As the critic Clement Greenberg explained in a 1958 tribute to Janis, the dealer's exhibition practices had helped to establish the legitimacy of the Americans, for his policy "not only implied, it declared, that Jackson Pollock, Willem de Kooning, Franz Kline, Phillip Guston, Mark Rothko, and Robert Motherwell were to be judged by the same standards as Matisse and Picasso, without condescension, without making allowances."
By the early 1930s, they had acquired major works by Picasso, Matisse, De Chirico, Dalí, Mondrian, and the self-taught master Henri Rousseau.
In 1939, as Chairman of MoMA's Art Committee, Janis helped arrange the loan of Picasso's Guernica to New York for the benefit of Spanish Refugee Relief.
The gallery soon acquired a strong reputation by mounting scholarly, curated exhibitions of Léger, Mondrian, the Fauves, the Futurists, and de Stijl artists.
Also in this decade, the gallery represented Arshile Gorky, Willem de Kooning, Franz Kline, Mark Rothko, Robert Motherwell, Phillip Guston, Adolph Gottlieb, William Baziotes, and Josef Albers.
The Sidney Janis Gallery soon became a leading exhibitor and dealer in Pop art, representing Claes Oldenburg, Jim Dine, Tom Wesselmann, George Segal, Öyvind Fahlström, and Marisol.
In 1967, he donated 103 works from his collection to the Museum of Modern Art, including six late Mondrian oils, Boccioni's Dynamism of a Football Player, and Picasso's Artist and Model.
In the final decade of the century, the Janis Gallery continued to mount significant exhibitions, including "Mondrian: Flowers" a rare gathering of an extensive group of floral images by the seminal abstract artist.
[5] During his lifetime, Janis continually sought to support art and creativity and create exposure for artists of his day even if they had not yet garnered the attention and adoration of the public.