1/1st

August 1914: in Brighton, part of the 1st South Eastern Mounted Brigade. Moved in 1915 to the Canterbury area.

September 1915 : dismounted, the Regiment moved to Gallipoli, landing on at Cape Helles on 7 October 1915 and becoming attached to the 42nd Division. The regiment was badly affected by disease, with many men suffering from enteritis within days of landing. Spent time in trench activity at Border Barricade and Fusilier Bluff.

30 December 1915 : evacuated from Gallipoli, arriving in Egypt in February 1916. At this time the Brigade was absorbed into the 3rd Dismounted Brigade. This formation was initially assigned to the Suez Defences and later moved to the Western Frontier Force.

3 January 1917 : formed the 16th (Sussex Yeomanry) Battalion, the Royal Sussex Regiment. This unit moved with it's Division to France in May 1918.

2/1st

Formed September 1914, and remained in UK until April 1918 when moved to Ireland. Converted into a cyclist unit in July 1916.

3/1st

Formed in 1915. Remained in United Kingdom until disbanded in January 1917.