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.

[5][6] By July 1916 the brigade was under the control of Southern Army of Home Forces, with its battalions billeted as follows:[15] Late in 1916 the War Office decided to form three new home-service divisions; 73rd was the last of these, assembling at Blackpool in November.

The division was based on 9th Provisional Bde, which moved from Margate and provided four infantry battalions and many of the support units.

[16][17] After assembling in Lancashire, 73rd Division moved in early January 1917 to join Southern Army of Home Forces, stationed in Essex and Hertfordshire, with the following composition:[16][17][18] Divisional Headquarters in Chelmsford.

Henceforth part of the role of the Home Service divisions was physical conditioning to render men fit for drafting overseas, alongside units of the Training Reserve.

[58] Between July and September 1917, four Graduated Battalions joined 73rd Division, and in October these were affiliated to line regiments and adopted territorial designations.

Only the divisional engineers remained, and on 22 June 1918 the HQ and field companies went to France for active service on the Western Front.