He led the Guides as part of Lord Methuen's force, sent to relieve the siege of Kimberley, fighting at the battles of Belmont (23 November 1899), Enslin, Modder River (28 November 1899), and Magersfontein (11 December 1899). Under Sir John French he commanded them at the battles of Paardeberg (February 1900) and Poplar Grove (7 March 1900), as part of the march to Pretoria, and then at the battle of Diamond Hill in June 1900. The focus of the operations in the Orange Free State then shifted to irregular guerrilla warfare, and Rimington's troopers saw heavy service here until the end of the year. Rimington left the Guides in January 1901. The force was reorganised as Damant's Horse under Major Frederic Damant, Rimington's second-in-command, though they were often known by his name through the remainder of the war.