On 4 October 1899, Sim was promoted to lieutenant-colonel, following which he was sent to South Africa in November as Commander Royal Engineers with the 5th Division, part of the Natal Field Force. Immediately on his arrival in South Africa, he led the Royal Engineers at the Relief of Ladysmith, including the operations of 17 to 24 January 1900, and the action at Spion Kop. This was followed by the operations on 5 to 7 February at Vaal Krantz and the Battle of the Tugela Heights from 14 to 27 February and the subsequent action at Pieters Hill, before the British finally defeated the Boers and entered Ladysmith on 1 March. Sim's actions during this phase of the Second Boer War earned him a brief mention in despatches from Lieutenant-General Sir Charles Warren.