A student at the University of Chicago, Cassady joined the United States Ambulance Service during Christmas vacation in 1916. Sailing from New York, he arrived at the front on February 3, 1917. After serving in the Champagne, Chemin des Dames and Argonne sectors, he enlisted in the French Foreign Legion. In July of 1917, he transferred to the French Air Service receiving his flight training at Avord, Tours, Pau and Plessis-Belleville. Breveted on 6 October 1917, Sergeant Cassady was assigned to Spa157 on December 26, 1917. In February of 1918, he joined the United States Air Service and after serving with the 103rd Pursuit Squadron, he was attached to Spa163 on May 14, 1918. The highest scoring ace in this escadrille, Cassady scored 5 victories as a SPAD pilot. On September 8, 1918, he was reassigned to the 28th Pursuit Squadron where he scored 4 more victories flying the SPAD S.XIII.

Cassady was promoted to Captain in March of 1919. As an officer in the OSS during World War II, he worked closely with the French underground until detained by the Germans in 1942. Released in 1944, he was sent to Algiers where he resumed undercover assignments in preparation for the Normandy invasion. With the liberation of Paris, Cassady returned to that city to serve as chief intelligence officer.

Distinguished Service Cross (DSC)
"For extraordinary heroism in action near Fismes, May 28, 1918 and near Epieds, France, June 5, 1918. On May 28, Lt. Cassady, single-handed, attacked an LVG German plane which crashed near Fismes. On June 5, as patrol leader of five Spads, while being attacked by twelve German Fokkers, he brought down one of the enemy planes near Epieds and by his dash and courage, broke the enemy formation." DSC citation

Distinguished Service Cross (DSC) Oak Leaf Cluster
"For the following act of heroism: on August 15, 1918, near Saint-Marie, while acting as protection for a Salmson, he was attacked by seven Fokkers, two of which he brought down and enabled the Salmson to accomplish its mission and return safely." DSC Oak Leaf Cluster citation

Légion d'Honneur
"He came to serve France at a time when there was no military obligation or compulsion. Object of a brilliant citation and gravely wounded in the Medical Section. He has since passed to aviation where he is indispensible in turn by the greatness of his character, his skill as a pilot, and his absolute disregard for danger. Officially credited with five enemy plances." Légion d'Honneur citation