At the outbreak of war, Sunbeam started to develop machines for use in the armed forces. The 3½ hp. machine was modified for service use and given an improved magneto drive, via an inverted Morse chain. A new gearbox was fitted with constant-mesh gears, and the fuel tank had three separate compartments, one contained petrol, another contained oil, and the third contained paraffin

The machines were finished in matt khaki with black lining and gold lettering. Sunbeam also produced some machines for the French army, which were the only Sunbeams produced with a belt drive.

In 1915 it was becoming difficult to obtain JAP engines, due to sales to the War Department, and so the 6 hp. machine was modified to accept a Sunbeam 6 hp. 73 mm. x 95 mm. twin engine.

The modified 6 hp. machines were also sold to the War Department, and were in service in Russia and Italy. Sunbeam also launched two new sidecars, 'Number 1' and 'Number 2'. These were built by Charles Hayward, who later joined A.J.S. He started in a loft at the Sunbeam works, but was soon forced to move due to lack of space.

Nine gold medals were won by Sunbeam in the 1915 Style Cop hill climb. This was mainly due to the efforts of George Dance on a newly developed 4 hp. single.

During 1916 large numbers of Sunbeam motorcycles were supplied to the Russian Army for use on the Eastern front. A new 8 hp. twin with a 3 compartment fuel tank was developed for this purpose. It was fitted with a machine gun and armoured sidecar. The machines were powered by a Swiss 996 c.c. MAG engine, and had Brampton 'Biflex' forks.

The sidecar was also produced as an ambulance (stretcher carrier) and there was even a double decker version. A 4 hp. and later 3½ hp. version was sold to the French army. These had a belt drive and were the only Sunbeams to be so equipped.

On 3rd November, 1916 the Ministry of Munitions suspended civilian production for the duration of the war, and so Sunbeam had to rely on W.D. contracts. In response to this the company stepped up production of vehicle and aircraft radiators which were needed in large quantities.

By 1918, JAP engines were available again and so the 8 hp. MAG engine was replaced with a JAP equivalent. This gave the machine an extra 4 inch ground clearance!  The motorcycle machine gun versions were unsuccessful as an effective weapon and so W.D. contracts for them were cancelled or terminated.

1918 was a disastrous year for the Marston family. John Marston's 3rd son, Roland, died at the early age of 45. He had been groomed as his father's successor at Sunbeam, and his untimely death came as a great shock to his parents. At the time John and his wife Ellen were staying at their house at Colwyn Bay. Unfortunately this was too much for John, who was overcome with grief. He died on the 8th March, the day after Roland's funeral. Sadly, Ellen also died six weeks later. John and Ellen were buried at Colwyn Bay.

The armistice was signed on 11th November and all fighting ceased. The Government immediately cancelled the surviving wartime contracts and ex-Sunbeam men returning from the war found themselves on a waiting list for their former jobs, which had been filled in their absence.