 Mosberg was born in Austria on June 14, 1896, but grew up on New York City's East side, a largely Jewish section.He began his prolific amateur boxing career in 1912 at age 16 with New York's Pastime Athletic Club and gained vast experience, eventually fighting an impressive 250 amateur fights.Boxing for the navy in World War I, Mosberg won the lightweight championship for his base, the Newport Naval Station at Newport, Rhode, Island, where he continued his amateur boxing career against navy fighters.