The regiment was formed on the creation of the Territorial Force in April 1908. It was headquartered at Church Street in Brighton, with the squadrons being headquartered as shown below, and under command of the South Eastern Mounted Brigade.
- A Squadron: Brighton (with drill stations at Horsham, Worthing, Haywards Heath and Crawley)
- B Squadron: Lewes (Burgess Hill, Eridge, Brighton, Uckfield, Tunbridge Wells and Haywards Heath)
- C Squadron: Chichester (Bognor)
- D Squadron: Eastbourne (St Leonards, Bexhill and Rye).
1/1st Sussex Yeomanry
- August 1914 : moved with the brigade to Canterbury.
- 24 September 1915 : dismounted, the brigade sailed on the “Olympic” from Liverpool to Gallipoli, landing on 8 October 1915 and coming under orders of 42nd (East Lancashire) Division.
- December 1915 : withdrew from Gallipoli and moved to Mudros.
- February 1916 : moved to Egypt where brigade was absorbed into 3rd Dismounted Brigade on Suez Canal defences.
- July 1916 : brigade moved to join the Western Frontier Force.
- 3 January 1917 : converted to infantry, becoming 16th (Sussex Yeomanry) Battalion of the Royal Sussex Regiment and came under orders of 230th Brigade in 74th (Yeomanry) Division.
- 7 May 1918 : landed at Marseilles and spent rest of war on Western Front.
2/1st Sussex Yeomanry
- Formed at Brighton in September 1914 as a “second line” (training, draft-supplying reserve) for the 1/1st.
- May 1915 : moved to Maresfield, came under orders of 2/1st Southern Mounted Brigade.
- October 1915 : moved with brigade to Canterbury, where brigade came under orders of 4th Mounted Division.
- July 1916: converted to a cyclist unit in 5th Cyclist Brigade, under 2nd Cyclist Division, and moved to Great Bentley (Essex).
- November 1916 : merged with 2/1st Surrey Yeomanry to form 8th (Surrey and Sussex) Yeomanry Cyclist Regiment, under command of 3rd Cyclist Brigade at Ipswich.
- March 1917 : resumed original identity but in same brigade. Moved to Bromeswell Heath (near Woodbridge) by May 1917. Moved in October to nearby Grundisborough.
- April 1918 : moved to Ireland, being initially at Dublin but soon to Clandeboye. In September 1918 moved to Boyle (Roscommon) where it then remained.
3/1st Sussex Yeomanry
- Formed as a “third line” (training, draft-supplying reserve for the 1/1st and 2/1st) in Brighton in July 1915.
- June 1915 : affiliated to 3rd Reserve Cavalry Regiment at Canterbury.
- January 1917: disbanded. The men were posted to either 2/1st Sussex Yeomanry or the 4th Reserve Battalion of the Royal Sussex Regiment.
In the 1917 renumbering of the Territorials, the Sussex Yeomanry issued numbers from the block 170001-175000. Corps of Dragoons.