Darell was commissioned into the Coldstream Guards as a second lieutenant on 8 September 1897, and promoted to lieutenant on 18 January 1899. He served in South Africa through the Second Boer War 1899–1902; where he took part in operations in the Orange Free State (April to May 1900), the Transvaal (May to June 1900, July to November 1900) and Cape Colony; and was present at several major battles, including at Belmont, Enslin and Modder River (November 1899), Magersfontein (December 1899), Poplar Grove and Driefontein (March 1900), Vet River, Zand River, Johannesburg, Pretoria and Diamond Hill (June 1900), Bergendal and Komatipoort (August 1900). From July 1901 he was signaling officer to a Mobile column. Following the end of the war, he return to the United Kingdom in August 1902.