He served in the First World War on the Western Front and in Mesopotamia. An intense individualist, Mason-MacFarlane's experience as a young artillery officer left him a reputation as an officer who was always ready to "look out for trouble" and who had a profound distrust of the higher command. A non-conformist, he was known for his love of amateur theatrical acting, sports and for writing sarcastic doggerel poems mocking the high command. In the winter of 1915–16, he served with the relief forces involved in the bloody fighting outside of Kut as the British sought to unsuccessfully rescue the Indian 6th Division besieged by the Ottomans inside Kut. He was awarded the Military Cross in 1916, a bar to the award in 1918 (gazetted in September) and a second Bar in the same year, awarded while he was attached to the Artillery Headquarters of the 41st Division, the most junior Kitchener's Army division. The citation for the second Bar was published in a supplement to the "London Gazette" on 2 December 1918, and reads: