The Second Boer War broke out in 1899, and in the following year McCalmont's battalion went to South Africa, serving in the Cape Colony and Orange River Colony. He was made a Companion of the Order of the Bath (CB) for his services in the war. A general election was held in 1900, and McCalmont, who was still in South Africa, was re-elected with an increased majority. His opponent in the election was C. D. Rose, owner of the racehorse Ravensbury which had been a rival to Isinglass.
