After passing out from the Royal Military College, Sandhurst, Evelegh was commissioned into the British Army as a second lieutenant in the Duke of Cornwall's Light Infantry (DCLI) on 1 May 1917, during the First World War. He served on the Western Front with the 1st Battalion of his regiment, a Regular Army unit that was part of the 95th Brigade of the 5th Division. He was promoted to lieutenant on 1 November 1918, just ten days before the war came to an end on 11 November 1918.