Guy Chapman was born in London in 1889. Educated at Oxford University he became a lawyer in 1914. Later that year, on the outbreak of the First World War, Chapman joined the Royal Fusiliers as a junior officer and arrived on the Western Front in August 1915.
After surviving the Battle of Arras in 1917, Chapman was badly affected by a mustard-gas attack. After treatment he returned to the Western Front and was still there when the Armistice was signed in 1918.