He practiced law until 1900, the same year he married Ethel Bliss (a future U.S. tennis champion) and decided to devote time to studying Renaissance art.
Dan Fellows Platt collected art for most of his adult life, beginning with Roman archaeological (coins, ancient glass, etc.)
His early collection was focused mainly on Italian Renaissance paintings from Siena and Umbria, inspired by Princeton professor Allan Marquand.
During this period, he collected work from Guercino, Luca Cambiasi, Théophile Steinlen, Alphonse Legros, John Flaxman, George Romney, and Amedeo Modigliani.
[5] Platt was a recognized expert on Italian Renaissance paintings, particularly from Siena, which led to multiple lectures at colleges around the country and being named chair of the Visiting Committee of the Department of Art and Archaeology at Princeton University.