LeRoy Jerome Prinz was born in St. Joseph, Missouri. His grandfather was a dancing master, and his father taught ballroom dancing etiquette to young men and women at Prinz's Academy in St. Joseph. According to one account, he was sent to reform school after chasing his stepmother with a carving knife.
In newspaper profiles, he claimed that after running away from boarding school at the age of 15, he "hopped a freight" and came to New York City, where, in 1911, he began a song and dance act, "Prinz and Buck," with a young black man he met along the way. Later that year, he told interviewers, he went on a ship to Europe as a cabin boy, jumped ship, and traveled around Europe "introducing the American strut step" in return for meals and lodging. In Marseilles he joined the French Foreign Legion, serving as a bugler in Algiers. He also represented a rubber company in St. Louis and Kansas City.
After the outbreak of World War I, he returned to France, was trained as a pilot, and served in the French aviation corps and Captain Eddie Rickenbacker's 94th Aero Squadron. He was with the 94th from November 1917 to June 1918, when he switched to the 27th Aero Squadron, where he stayed until November 1918. At the 27th his duties included working at the Aircraft Acceptance Park test facility at Orly. Prinz subsequently told journalists that he crashed 14 to 18 airplanes, was nicknamed "America's German Ace" as a result, (he was also called "Crash Ace Prinz) and that he was wounded in the war and carried a silver plate in his head from his last plane crash. In an October 1918 article, war correspondent George Seldes described how Prinz was separated from his flight on his first venture into German territory, and returned home with minor injuries after a perilous journey.
According to his obituary in the Los Angeles Times, Prinz returned to the U.S. in 1919 and studied theater at Northwestern University. After graduation from Northwestern, the newspaper reported, Prinz returned to France and worked as a choreographer for the Folies Bergère in Paris.