Munn was born October 17, 1906, in Prescott, Arkansas, and after graduating from high school in 1923, he attended the United States Naval Academy. After graduating on 2 June 1927, he was commissioned as a second lieutenant, he attended Officers' Basic Course at the Philadelphia Navy Yard and was assigned to the 2nd Marine Brigade to participate in the United States occupation of Nicaragua. Returning to the United States in September 1929, he was assigned to the guard force at Herbert Hoover's summer camp near Criglersville for three months, then was ordered to attend aviation training at Hampton Roads.