William Larimer Mellon Sr. (June 1, 1868 – October 9, 1949), sometimes referred to as W. L., was an American businessman who was active in Republican Party politics. A co-founder of Gulf Oil, he was a member of the prominent Mellon family of Pittsburgh, Pennsylvania. According to The Miami News, "Politically, Mellon was known as the 'field marshal' for the Mellon political forces in the days preceding the New Deal.