After the outbreak of the Second Boer War in late 1899, Brassey volunteered for active service and was commissioned Captain of 69 (Sussex) Company of the Imperial Yeomanry on 28 March 1900.
[4] He left Southampton for South Africa in early April 1900 on the SS Carisbrooke Castle, accompanied by his wife.
[5] While in South Africa he served as acting Civil Commissioner for the British government at Pretoria in 1901, following its surrender by the Boers the previous year.
[2] After the outbreak of the First World War he raised a second battalion to this regiment[3] for home service, which he commanded until 1916, remaining on the Territorial Force Reserve.
[3] In the latter year's general election, he fought against Kenneth Robert Balfour, and after a close and disputed result, the two men gave their names to adjoining roads in the constituency.