Charles Mayer was born in Molsheim during one of the brief periods in which it was part of Germany (it is now part of France). By 1900 he had immigrated to New York with his family and was working as a plumber, while training on the side as an amateur boxer. He became the only boxer from the 1904 Olympics to have won a U.S. national championship in another year. After winning the Olympic middleweight championship in 1904, he added the 1905 AAU middleweight championship. Mayer fought in the heavyweight division in 1904 only to appease the crowd who wanted to see him fight the heavyweight champion, Sam Berger. Though outweighed by 22 lbs. (180-158), Mayer put up a good fight but lost by decision. He eventually moved to California, where he ran his own plumbing business.