On 12th March 1915, Ferris enlisted into the 18th Bn. RIR, aged 33, before being promoted to 2nd Lieutenant. He was posted to the 12th Bn. Royal Irish Rifles and then transferred to the Machine-Gun Corps. Ferris saw service with the British Expeditionary Force and was later promoted to Lieutenant. He saw action in the Somme 1916 (Thiepval), Arras (Hindenburg Line) and the Allied advance 1918. Demobilisation came on 22nd March 1919 and he was awarded the 1914/15 Star, the British Victory Medal and the British War Medal.
The Staff Magazine ‘The Link (August 1981)’ contains a photograph of Ferris in a 1913 Northern Bank Staff Golf outing.