With the outbreak of the First World War, Barttelot initially served in France with the 4th (Reserve) Battalion Coldstream Guards, from 12 August 1914, where he was wounded at the Battle of the Aisne and also awarded the French Croix de guerre. He was then appointed a General Staff Officer, Grade III on 11 February 1915 and was concerned with coastal defence around Portland, and served in the ill-fated Gallipoli campaign from July 1915, where he was promoted major on 1 September 1915. He was appointed a Brigade Major on 11 January 1916, for the Mesopotamian campaign. He was appointed to the Distinguished Service Order (DSO) on 22 December 1916, and appointed GSO, Grade II from 11 September 1917. and was Military Attaché to Tehran from 17 March 1918 with temporary promotion to lieutenant-colonel whilst performing those duties. He was awarded a brevet promotion to lieutenant-colonel for his service in Mesopotamia on 3 June 1918, and was killed, (aged 38) on 23 October 1918 in Tehran, Persia and was buried in the Tehran War Cemetery. In addition to his DSO, he had also been Mentioned in Despatches four times.