He began his professional music career playing bass clarinet for the New Haven Symphony Orchestra while attending Yale. Leyden joined the United States National Guard in 1940 for a planned year of volunteer work and enlisted as an infantry sergeant on February 24, 1941, in New Haven, Connecticut. His enlistment papers give his height as six foot two and his weight as 165 and give his specialty as a musician or band leader. During World War II he served in the Army Air Force for five years.