August 1914 : in Windsor. On mobilisation, a Squadron transferred to the Composite Regiment. 1 September 1914 : attached to 7th Cavalry Brigade, 3rd Cavalry Division. 21 November 1914 : transferred to 8th Cavalry Brigade, 3rd Cavalry Division. 7 November 1917 : transferred to 7th Cavalry Brigade, 3rd Cavalry Division. 10 March 1918 : left Brigade, and converted into No 3 (Royal Horse Guards) Bn, Guards Machine Gun Regiment. 24 May 1918 : transferred to First Army.