A scientist with a PhD in plant physiology, Marshall became independently wealthy after the death of his father in 1929. He had started his outdoor career in 1925 as forester with the U.S. Forest Service. He used his financial independence for expeditions to Alaska and other wilderness areas. Later he held two significant public appointed posts: chief of forestry in the Bureau of Indian Affairs, from 1933 to 1937, and head of recreation management in the Forest Service, from 1937 to 1939, both during the administration of President Franklin D. Roosevelt. During this period, he directed the promulgation of regulations to preserve large areas of roadless land that were under federal management. Many years after his death, some of those areas were permanently protected from development, exploitation, and mechanization with the passage of the Wilderness Act of 1964.