The son of a career Army officer, Ewell graduated from the New Mexico Military Institute and the United States Military Academy. Commissioned as a second lieutenant of infantry in 1939, he volunteered for paratrooper training at the start of World War II. During the war, he commanded 3rd Battalion, 501st Parachute Infantry Regiment, part of the 101st Airborne Division. He took part in a parachute jump into Normandy during the D-Day invasion, and continued to take part in combat against the Nazis in Europe. Ewell later commanded the 501st Regiment, which included participation in Operation Market Garden and the defense of Bastogne in the Battle of the Bulge. He received the Distinguished Service Cross for his heroism at Bastogne.