During World War I, Mahan served as a first lieutenant with the U.S. Marine Corps. At the end of the war, Mahan remained in Europe for a time working for Herbert Hoover's Food Administration. In a January 1919 game played in France, Mahan led a Navy football team to a 12–0 win over a U.S. Army team. A press account reported that Mahan was the star of the game, running 65 yards for the first touchdown:"In spite of the muddy field, speed and generalship on the part of the S.O.S. eleven, largely in the person of Eddie Mahan, … told the tale. It was the tall, black-haired Mahan with an undershot jaw that broke the deadlock in the second period. He grabbed a short punt …, eluded two men who barred his path, shook off a third tackler further down the field, and sprinted 65 yards for a touchdown."