 In 1940, he became head coach at St. Ambrose University in Davenport, Iowa, where he led the football team to an undefeated season.After American entry into World War II, Mullins entered the United States Navy Reserve.He was sworn in as a lieutenant senior grade in Jacksonville, Florida on March 23, 1942, and then attended a month-long course in Annapolis, Maryland prior to service in the Navy's physical training program.