 He simultaneously worked as a navy boxing instructor, as did his friend boxer Mel Coogan, before he was discharged and returned to his civilian amateur career.His patriotic service in World War I and subsequent training and participation in the Olympics delayed his entry into boxing's professional ranks until he reached the age of twenty four.Mosberg did not perform well enough in the Olympic trials in Boston to be placed on the two-person American lightweight boxing team, but was picked as an alternate and sailed to the Olympics in Antwerp, Belgium in August 1920.