Driving a Mercedes, Wagner finished second to Christian Lautenschlager in the 1914 French Grand Prix at Lyon. He competed in the 1919 Indianapolis 500 driving a Ballot but went out with a broken wheel on lap 45. In 1924 he drove for the Alfa Romeo team, in a P2 alongside Antonio Ascari and Giuseppe Campari. In 1926, Wagner teamed up with Robert Sénéchal to drive a Delage 155B to victory in the first ever British Grand Prix. In August he won the Grand Prix de la Baule in a Delage 2LCV. In addition to Grand Prix racing, Wagner also competed in the 24 hours of Le Mans endurance race.