Durnford served as an Ensign in the British army Corps of Royal Engineers, having signed up on 17 March 1759. In the Seven Years' War he participated in the Capture of Belle Île, France, 1761 and was also at the Battle of Havana (1762). After the Cuban action Lord Albemarle made him an Aide-de-camp. Durnford became an accomplished artist while on the Cuban campaign and engravings made from his sketches are highly valued.