The troops at Newhaven Fort were regular soldiers of the Royal Garrison Artillery, but Newhaven Fort was also used to train volunteers - part-time soldiers who became known as Territorials in 1908.

When the Great War started in August 1914, the Territorials took over the manning of Newhaven Fort from the peacetime garrison regulars. In addition to the gunners of the Sussex RGA, there were Territorial Royal Engineers manning searchlights out on the harbour breakwater and an infantry battalion of the Royal Sussex Regiment, although the later were not accommodated in the fort, and were encamped a short distance away at Meeching Rise.

By 1914, a considerable amount of modernisation had taken place at Newhaven Fort. The old style muzzle-loading guns were replaced by up-to-date breech-loading types with a much higher rate of fire and the gun positions altered or rebuilt to suit the new weapons. In 1907, a newly built battery observation post was fitted with the modern telephone and had a rangefinder from which information could be transmitted directly to the guns. Voice communication by speaking tube was also provided. The soldiers’ living quarters had been improved; four baths had been added and a 'recreation room' was established.

During the First World War Newhaven harbour became the main military supply port for the British Expeditionary Force in France, as well as being a base for naval vessels on escort and other duties in the English Channel. With this degree of military importance, the area was well defended. The fort's 6" guns were manned to work in association with a naval examination vessel, operating in the vicinity of the harbour mouth. This vessel's responsibility was to challenge and inspect all shipping approaching the harbour. If its order to stop was ignored, a fort gun would put a shot across the bows of the offender.

Although an assault on the port of Newhaven was always considered as a military possibility, it never became a reality and life for the Newhaven Fort garrison was uneventful. With heavy casualties on the Western Front, fit men were posted away from the fort to more active service.

When the Great War ended, the garrison consisted largely of troops in low medical grades. With peacetime, Newhaven Fort became almost deserted. A Master Gunner with one NCO and a couple of civilian labourers were required only to keep the place in good order and service the guns.