After six years in the army reserve with the Australian Light Horse, Green enlisted in the military at the outbreak of World War I and embarked as a lieutenant in the 6th Light Horse Regiment on the "Suevic" on 21 December 1914. He saw service in Egypt, at the Battle of Gallipoli and on the Western Front; he was shot once at Gallipoli, recuperated with a long hospital stay and return to Australia in 1916, before re-embarking in November 1916. He served until again injured when he lost a leg at the Battle of Menin Road in Belgium, in September 1917. He returned to Australia on 30 June 1918 and returned to his farm at Woolomin. He was a co-founder and the first vice-president of the Limbless and Maimed Soldiers' Association, a member of the Tamworth Pastures Protection Board, and was an unsuccessful Progressive Party candidate at the 1922 state election.