He was serving with the Western Universities Battalion at the start of World War I, soon afterwards became a non-commissioned officer and went with them to France in 1916.
He was severely wounded by machine gun fire during a raid near Vimy Ridge and was awarded the Military Cross.
In 1940, during World War II, he was posted to Washington, D.C. as military attaché to the Canadian joint staff mission.
Letson helped conceive the CANLOAN scheme while visiting Canadian Military Headquarters in London in October 1943.
[6] The Letson Trophy is awarded to the winning team in the Canadian Forces Small Arms Concentration.
[2] The Letson Prize is awarded annually in equal amounts to the head of the graduating class in each option of the undergraduate mechanical engineering program.