Career
Riordan joined the Army from Kansas City, Missouri in 1940, and by February 3, 1944 was serving as a second lieutenant in the 133rd Infantry Regiment, 34th Infantry Division. On that day, near Cassino, Italy, Riordan single-handedly silenced a German machine gun emplacement. Five days later, on February 8, he was again in the lead during an attack on a German strongpoint.
Cut off from his unit, he attempted to take the objective on his own, but was killed in the process.
Riordan, aged 23 at his death, was buried in Mount Olivet Cemetery, Raytown, Missouri.