With the outbreak of the Second World War, he interrupted his studies in music to serve in the British Army. He was commissioned as a second lieutenant in the Duke of Cornwall's Light Infantry (DCLI) on 15 February 1941, and was awarded the Military Cross as a temporary captain for his actions during the Battle of Normandy on the night of 10/11 July 1944, when he was serving with the 5th Battalion, DCLI as battalion intelligence officer. The battalion, part of the 214th Infantry Brigade of the 43rd (Wessex) Infantry Division, was ordered to hold Hill 112 in Normandy, France, as part of Operation Epsom. He carried out his duties outstandingly overnight, helping inflict severe casualties on the German forces by calling in artillery support to break up counter-attacks. The battalion suffered over 250 casualties during the night, including the commanding officer and one of the company commanders. This left Willcocks in command of the battalion headquarters, which by then was the furthest forward part of the battalion. He rallied the men, enabling the battalion to stand firm and reorganise. The award was gazetted on 21 December 1944.