After the outbreak of the Second Boer War in October 1899, MacCormac volunteered to go out to South Africa as a consulting surgeon to the forces, and from November 1899 to March 1900 he saw much active service both in Cape Colony and Natal. He accompanied Lord Roberts on his visit to Kimberley after a siege of that city had been lifted in February 1900, and the following month visited Ladysmith when that city was relieved from a siege. He left South Africa on board the German liner "Kaiser" in mid-March 1900. His assistance to the British war effort was acknowledged on his return when he was appointed a Knight Commander of the Order of the Bath (KCB) for his services.