Clark was an avid art collector, with voluminous holdings ranging from Old Masters, French Impressionists and Early Modernists to American colonial portraits, Hudson River School works, American genre paintings and American folk art. As a young man, he helped to organize the 1913 Armory Show, and purchased the most expensive sculpture in the exhibition: Wilhelm Lehmbruck's "Standing Woman" (later donated to MoMA). He acquired 13 works by Matisse by the early 1930s, which he exhibited in the "Matisse Room" of his city house. He sold or gave away the Matisses prior to his death. Clark assembled a choice collection of works by American painter Thomas Eakins, some purchased from the artist's widow, which he bequeathed to the Yale University Art Gallery, the Addison Gallery of American Art, and the National Gallery of Art in Washington, D.C. The 40 paintings he left to Yale also included works by Frans Hals, Van Gogh, Manet, Winslow Homer and George Wesley Bellows. His bequests to the Metropolitan Museum of Art included paintings by Renoir, Degas, Seurat and Cezanne, along with a $500,000 cash bequest to install air-conditioning and smoke detectors in its Fifth Avenue building.