On the outbreak of World War I the Territorial Force (TF) immediately mobilised for home defence, but shortly afterwards (31 August 1914), its units were authorised to raise 2nd battalions formed from those men who had not volunteered for, or were not fit for, overseas service, together with new volunteers, while the 1st Line went overseas to supplement the Regulars. Early in 1915 the 2nd Line TF battalions were raised to full strength to form new divisions, and began to form Reserve (3rd Line) units to supply drafts. The remaining Home Service men were separated out in May 1915 to form brigades of Coast Defence Battalions (termed Provisional Battalions from June 1915).
During the autumn of 1915, 5th Provisional Brigade was stationed in Suffolk, attached to 58th (2/1st London) Division for administration, and to 1/1st Mounted Division for operations. In April 1916 2/1st East Anglian (Essex) Heavy Battery Royal Garrison Artillery joined the brigade and remained with it until the end of the war. By July 1916 the brigade was under the control of Northern Army of Central Force, with its battalions billeted as follows:
The Military Service Act 1916 swept away the Home/Foreign service distinction, and all TF soldiers became liable for overseas service, if medically fit. The Provisional Brigades thus became anomalous, and at the end of 1916 their units became numbered battalions of their parent units. Part of their role was physical conditioning to render men fit for drafting overseas. 5th Provisional Brigade became 225th Mixed Brigade in December 1916, with its units redesignated as follows:
In May 1918 each of the Mixed Brigades was called upon to provide a battalion (redesignated a Garrison Guard battalion) to reconstitute the 59th (2nd North Midland) Division, which had been virtually destroyed during the German Army's Spring Offensive. 225th Mixed Brigade supplied 15th Essex to 177th (2/1st Lincoln and Leicester) Brigade and immediately raised a new 18th (Home Service) Battalion Essex Regiment to take over its coast defence duties. The brigade remained with this composition until the end of the war, after which it was demobilised.