At the outbreak of the First World War, Green was commissioned into the Royal Army Medical Corps. The early part of his military career was spent attached to the South Staffordshire Regiment as a medical officer before being transferred to the Field Ambulance. He was later posted to the Sherwood Foresters with which he went to France as part of the 46th (North Midland) Division, which fought in the Battle of Loos. His brother, who served in the South Staffordshire Regiment, was killed in the battle.