Complex sentence: However, his longest association was with the Thanhouser Film Corporation of New Rochelle, New York, for whom he worked, usually as an actor but sometimes as a director, from 1914 to 1917, including "The Picture of Dorian Gray" (1915).
Split into simpler sentences: However, his longest association was with the Thanhouser Film Corporation of New Rochelle, New York, for whom he worked, usually as an actor but sometimes as a director, from 1914 to 1917. For example, Cooper appeared in Thanhouser's "The Picture of Dorian Gray" (1915).