Cartland achieved the rank of major in the British Army. In February 1937, he was commissioned in the Territorial Army. By August 1939, he was a lieutenant in the Worcestershire and Oxfordshire Yeomanry. When the Germans commenced the French and Low Countries campaign in May 1940, he was serving in the 53rd Anti-Tank Regiment (The Worcestershire Yeomanry), Royal Artillery. The unit was assigned to defend the town of Cassel, a hilltop site near one of the main roads leading to the Channel port of Dunkirk, France. Cartland and his men held off the Germans from 27 to 29 May. On the evening of 29 May, Cartland and his unit split up and joined the retreating British Expeditionary Force, which was heading towards Dunkirk. On 30 May 1940 during the Battle of Dunkirk, while he was reconnoitering his position from a ditch, he was shot and killed near Watou, Belgium.