Encouraged by Frick, and guided by Charles Carstairs. Mellon began collecting art in the mid-to-late 1890s. Over the ensuing decades, his art collection continued to grow as he purchased pieces from Knoedler and Joseph Duveen, and during the 1920s his collection was generally considered to be one of the finest art collections in Washington. In the early 1930s, he purchased, according to Cannadine, just under half of the "greatest paintings" from the Soviet Union's Hermitage Museum in the Soviet sale of Hermitage paintings. Twenty-one paintings, of the highest quality, were later given to the National Gallery of Art.