The regiment was formed on the creation of the Territorial Force in April 1908 and placed under orders of the 2nd South Western Mounted Brigade. It was headquartered at the County Territorial Hall in Taunton with the squadrons being headquartered as follows:
- A Squadron: Wellington (with drill stations at Minehead, Wiveliscombe, Washford, Dulverton and Williton)
- B Squadron: Taunton (Churchinford, Buckland St Mary, Bishops Lydeard, Churchstanton and Hatch Beauchamp)
- C Squadron: Bridgwater (Highbridge, Glastonbury, Langport, Nether Stowey and North Petherton)
- D Squadron: Yeovil (Crewkerne, Chard, Ilminster, South Petherton and Martock).
1/1st West Somerset Yeomanry
- August 1914 : moved with brigade to Winchester after mobilising but by 15 August had moved on to Ardleigh (near Colchester). It moved on in October to Great Bentley and in November went to Tendring. By September 1915 it had moved again, to Thorpe-le-Soken.
September 1915 : sailed from Liverpool on the “Olympic” for dismounted service at Gallipoli.
- 9 October 1915 : landed at Suvla bay, whereupon brigade came under orders of 11th (Northern) Division. It was later, while still at Gallipoli, transferred to 2nd Mounted and then 53rd (Welsh) Divisions.
- December 1915: evacuated from Gallipoli and moved to Egypt.
- February 1916 : placed under orders of 2nd Dismounted Brigade in Western Frontier Force.
- 4 January 1917 : now at Moascar, converted into infantry and became 12th (West Somerset Yeomanry) Battalion of the Somerset Light Infantry. Placed under orders of 74th (Yeomanry) Division.
- 7 May 1918 : division landed at Marseilles for service in France.
2/1st West Somerset Yeomanry
- Formed as a Second-Line regiment in September 1914, coming under orders of 2/2nd South Western Mounted Brigade in January 1915. Moved to Woodbury (Essex) in September 1915. Brigade was later renamed as 2nd Mounted Brigade.
- May 1916 : brigade came under orders of 1st Mounted Division in Norfolk.
- July 1916 : division became 1st Cyclist Division. Regiment was converted to a cyclist unit, moved to Beccles and came under orders of 1st Cyclist Brigade.
- November 1916 : merged with 2/1st City of London Yeomanry to form 5th (West Somerset and City of London) Yeomanry Cyclist Regiment, under orders of 2nd Cyclist Brigade.
- March 1917 : now near East Dereham, still in same brigade.
- May 1918 : moved with brigade to Ireland, stationed at Athlone where it then remained.
3/1st West Somerset Yeomanry
- Formed as a Third-Line training unit around May 1915.
- Details found in men’s service records suggest it was at Aliwal Barracks, Tidworth by late 1915.
- July 1916 (although possibly a month earlier): attached to 3rd-Line groups of Wessex Division at Winchester.
- February 1917: disbanded, with troops going partly to 2/1st Regiment and rest to 4th Reserve Battalion of Somerset Light Infantry at Bournemouth.
In the 1917 renumbering of the Territorials, the West Somerset Yeomanry issued numbers from the block 295001-300000. Corps of Hussars.