The following year, 1900, he was recruited by representatives of the University of Michigan, who saw him win a race at a county fair. At the 1904 Olympic Games in St. Louis, Hahn earned three gold medals. He began by winning the 60m dash in 7.0 seconds. Next, he won the 200m, leading the final from start to finish, in a time of 21.6 seconds. Finally, Hahn shot out to a fast start in the 100m final and held on to defeat fast-finishing Nate Cartmell by about two metres. Running into a strong wind, Hahn's time was 11.0 seconds.
Two years later, Hahn traveled to Athens for the 1906 Intercalated Games. Taking advantage of his usual quick start, he led the final from start to finish and beat fellow American Fay Moulton by one metre. Hahn studied law at university, but never practiced his profession. Instead he devoted his life to coaching young runners. His book How to Sprint was considered a classic text.