Frew joined the Highland Light Infantry in 1914. After serving in France, he transferred to the Royal Flying Corps in August of 1916. He was assigned to 45 Squadron on April 28, 1917. Frew served in France and Italy and was credited with 23 victories while flying the Sopwith 1½ Strutter and Sopwith Camel. In 1918 he was injured when his Camel was hit by anti-aircraft fire. He returned to England and served as an instructor for the remainder of the war. Frew was knighted and retired from the Royal Air Forcewith the rank of Air Vice-Marshal in 1948.