Roberts attended the Royal Military Academy, Woolwich, the British Army's officer training centre for the technical corps. He received a commission as a second lieutenant in the Royal Engineers on 26 August 1916. He took part in the 1917 Palestine campaign and fought on the Western Front in 1917–1918. He was promoted to lieutenant in February 1918. After the war, in 1919 he took part in the North Russia intervention, a military action carried out by the Allies during the Russian Civil War. He was awarded a Military Cross for gallantry in this campaign. Roberts also played rugby union for the army. After Russia Roberts returned to England, where he resigned his commission on 24 December 1919, transferring to the General Reserve of Officers.