Wagner was one of the drivers for the Darracq team in the 1904 Gordon Bennett Cup in Germany that finished 8th and in 1905 at the Circuit d'Auvergne in Clermont-Ferrand, he was eliminated in the first round.

Competing in the United States, Wagner won the Vanderbilt Cup of 1906 driving a Darracq model 120 over a Long Island racecourse.

He finished fifth in the 1907 Kaiserpreis in Germany but the following year in Savannah won the first ever United States Grand Prix driving a Fiat.

In addition to Grand Prix racing, Wagner also competed in the 1925 24 Hours of Le Mans together with fellow countryman Charles Flohot in a Ariès Type S GP, they finished sixth overall, and second in class 3.0.

In 1955 Wagner was awarded the Legion d'Honneur for distinguished service in the First World War, although it had been delayed 37 years as a criticism of his racing for Mercedes-Benz shortly before that conflict began.