On the outbreak of the First World War Gill joined the British Army and in 1915 was sent to France as a member of the Royal Artillery. The following year he was seconded as a camouflage officer in the Royal Engineers. Gill was recruited as an official war artist in 1918 and all his paintings on the war were completed after the Armistice.
In the 1920s and 30s Colin Gill concentrated on decorative murals. It was while working on a public mural in Johannesburg, South Africa that he died in 1940.