The outbreak of war in 1914 saw National Reservists on parade in the Borough on Bank Holiday Monday, 4 August, prior to proceeding to Taunton to receive their colours - war with Germany was declared at midnight. Later recruits, in response to Lord Kitchener's appeal, marched through Middle Street to entrain at Yeovil Town Railway Station, and horses were commandeered for army use.

Sir Ernest Petter, in 1915, offered the Petter works to the government for use in assisting the war effort. The Admiralty being in need of seaplanes, a new episode in the company’s affairs was opened, the Westland works being given over for the construction of aircraft - an activity which continued after the end of the war. The Nautilus factory in Reckleford was employed in producing munitions.
In 1915 the British Red Cross Society, took over the Newnam Memorial Hall in South Street for use as Yeovil Red Cross Hospital, equipping it with four wards, a recreation and dining room, ancillary offices, and a fully equipped operating theatre. Some 1,200 war casualties were treated here until its closure in December 1918.

In World War II Yeovil suffered enemy air raids on ten occasions, 107 high explosive bombs fell on the town killing or fatally wounding 49 persons, a further 32 were seriously injured and 90 slightly. Sixty-eight houses were totally destroyed, 2377 were damaged but repairable, 309 had windows broken, and 67 were damaged by machine gun fire - a total of repairable buildings of 2754 - accounting for a third of the borough’s houses. Fire bombs, however, did not fall on the town.

The first, and most serious, raid took place on 7 October 1940 followed by another on the following day. These caused 27 fatal casualties, two bombs making direct hits on air-raid shelters in Preston Road and at the Vicarage Street Methodist Church. The final raid occurred on 5 August 1942, two bombers flying over the town just before dusk and inflicting damage on nearly a thousand houses.