In 1942, Simpson enlisted in the U.S. Army. He served as a Captain, then Major, in Army intelligence, Simpson served with American forces in North Africa and western Europe until 1944. Then he resigned, suffering from a severe bout of hepatitis. He returned home, with two Bronze Stars. He was promoted to Curator (head) of the Department of Geology and Paleontology at the American Museum of Natural History. He also held a post as Professor of Zoology at Columbia University (19451959). He took up his work on early mammals, and worked on the Palaeocene and Eocene faunas of the San Juan Basin, New Mexico. 