Due to the American entry into World War I in April 1917, Corlett worked in the early organization and expansion of the Signal Corps and was its first commanding officer and executive. He served on the Western Front where, as Director of Signal Corps Supplies for the American Expeditionary Force (AEF), he was injured by mustard gas while laying communication lines at the front. He crossed the Rhine into Germany with the first American troops at Coblenz. He was promoted to lieutenant colonel in 1918 at the very young age of 29. The war ended on November 11, 1918. For his service during the war he was awarded the Army Distinguished Service Medal.