Clyno built large numbers of motorcycles for the British Army throughout the First World War. The majority were sidecar outfits designed to carry a Vickers machine gun. Each carried ammunition, cooling water and spares for the gun together with petrol and spares for the motorbike. An outfit like this would have been accompanied by two others; one carried extra ammunition and the other was a spare gun carrier, brought into use if the first bike broke down. At the Armistice in 1918 the British armed forces had 1,792 Clyno motorcycles, 1,150 at home and 642 overseas, including 478 in France.

Clyno also signed an agreement with the Russian war commission to supply their army on top of their business with the British forces.