At the onset of the First World War the Special Reserve of the Royal Army Medical Corps was mobilised and in December 1914 Elliot was attached as a medical officer to the Royal Scots Greys on the Western Front. He won a Military Cross for his actions at Wancourt during the Battle of Arras in April 1917. He won a second Military Cross in the Battle of Cambrai in November 1917, adding a bar to the original medal. Walter received a leg wound in the final month of the war, but returned home safely. His younger brother Dan Elliot was killed in 1915, during the Gallipoli campaign.