Too young for military service at the outbreak of the Second World War, Boscawen went to Trinity College, Cambridge, where he read mechanical science and took the special army engineering course. In 1941, he joined the Royal Engineers. However, on 4 September 1942, he was commissioned as a second lieutenant into the 1st Battalion of the Coldstream Guards (with which members of his family had served since 1769, including his brothers George and Evelyn, who had been killed during the evacuation from Dunkirk), and his service number was 243507. The battalion formed part of the 5th Guards Armoured Brigade, part of Major General Allan Adair's Guards Armoured Division, and Boscawen was sent to the cavalry wing of Sandhurst to train as a tank commander. In September 1944, after having fought in the Battle of Normandy, his battalion were among the first tanks to enter Brussels. A week after Operation Market Garden had finished, he was awarded the Military Cross (MC) during the German counter offensive against the Nijmegen bridgehead.