Joseph H. Chapin, the art editor of Scribner’s Magazine, discovered one of Clark's drawings on a classroom wall and gave him his first commission, to illustrate a story by Rudyard Kipling. In 1899, after achieving rapid success illustrating books and magazine stories, Clark returned to the Art Student League as a teacher. He also taught briefly at the Pennsylvania Academy of the Fine Arts. Clark won medals at the 1900 Paris Exposition and the Pan-American Exposition of 1901. In New York, he shared a studio with the writer Guy Wetmore Carryl and illustrated two of his stories.