Barker, by now having gained the nickname of "Bubbles", was still in India with his battalion when the First World War began in August 1914. However, in November, the battalion was sent to England where it became part of the 80th Brigade of the 27th Division. On 27 November Barker was promoted to the temporary rank of lieutenant. The following month the 27th Division was sent to reinforce the British Expeditionary Force (BEF) on the Western Front, where Barker remained until being wounded in March 1915 during the actions of St Eloi Craters. By now recovered from his injuries, Barker returned to his battalion, which in November 1915 was sent to the Salonica Front, where he would remain for the rest of the war. Promoted to the rank of captain on 10 April 1916, he became the battalion's adjutant, remaining in this post until August 1917, when he became a General Staff Officer Grade 3 (GSO3) on the HQ of the 22nd Division, a Kitchener's Army formation. He relinquished this appointment on 11 January 1918 upon becoming a brigade major with the 67th Brigade. He ended the war in November, having been wounded twice, awarded the Military Cross (MC) on in June 1917 and the Italian Silver Medal of Military Valor in September, the Distinguished Service Order (DSO) on 3 June 1919 and was twice mentioned in despatches, once on 21 July 1917 by Lieutenant General Sir George Milne, Commander-in-Chief (C-in-C) of the British Salonika Army, and again on 5 June 1919.