Forces aériennes françaises libres
Cornthwaite began his time in the US Army Air Force as a radio-gunner in B-25 aircraft as part of the 12th Medium Bombardment Group in 1942. He then served as a radio operator and superintendent of the watch with the 79th Fighter Group from 1943-1944. This was followed by serving as a writer and broadcaster with the Mediterranean Allied Air Forces from 1944 until 1945. In this position he performed Public Relations duties in Naples, Rome, Corsica and other locations in the Mediterranean area. While on Corsica, Cornthwaite worked on his first film which was for the Free French Air Force. He acted as a writer and sound engineer recording Capitaine Eve Curie (daughter of Marie and Pierre Curie) of the Free French Air Force. He did the commentary in French and English. Cornthwaite also dubbed the voice of American Air Force General Ira Eaker since he did not speak any French.