Turnbull's Welsh Guards company had pushed forward of its supporting tanks during a Panzer counter-attack and was cut off in the bocage countryside south of Montchamp. Turnbull went forward on a personal reconnaissance to plot a way back to the battalion. A German sniper concealed in a hedgerow shot him through the head; he died instantly.
His body was recovered after the position was retaken later that day by Sergeant Fred Llewellyn, one of his own men, who carried Turnbull's personal effects back through the Welsh Guards lines and, eventually, to his widow Elizabeth in Glamorgan. The Welsh Guards lost more than 100 men in the fighting around Montchamp on 5 August 1944.
A county captain who had filled almost every role available to a 1930s amateur — Cambridge Blue, England middle-order batsman, Glamorgan secretary, Welsh rugby international, England squash player — Turnbull was already secretary of his county and a member of the MCC committee when he joined the Welsh Guards in 1939. His wartime Glamorgan duties had been delegated to JC Clay; cricket assumed he would resume as captain in 1946.