Charles McGarel Johnston received his schooling at St Columbia College, near Dublin, and Cheltenham College, England. He came to South Africa in May 1894, aged just 18. During the next few years he travelled over most of southern Africa south of the Zambesi River, and worked in various positions. He saw military service in present Zimbabwe as a trooper of the British South Africa Company during in the Matabele rebellion (1896), for which he was awarded a medal, and as a lieutenant in the Fifth Rhodesian Volunteers during the Anglo-Boer War (1899-1902), earning the King's and Queen's medals with bars.
After the war he was appointed in the Department of Agriculture of the Orange River Colony (now the Free State) and from December 1905 was temporarily in charge of the department's biological division. He wrote a "Report on some birds in the Orange River Colony in their relation to agriculture", which was published as part of the report of government entomologist W.R. Dewar*, in the department's Annual Report for 1904/5 (pp. 221-228). He also wrote pamphlets on parasites of poultry, sheep scab, and the destruction of vermin. By 1907 he had been appointed chief locust officer of the Orange River Colony. Meanwhile he also farmed near Winburg and was learning Dutch. In July 1909 he visited Rhodesia (now Zimbabwe) as one of the three members of an official delegation sent there from the Orange River Colony to investigate farming possibilities. The resulting Report on Rhodesia... was written by Johnston in consultation with the other delegates and published in Bloemfontein in December that year. At some time he became secretary to the Central Agricultural Society of the Orange Free State (established in 1909) and the Friesland Cattle Breeders' Association.
Johnston's hobby was ornithology. He became a member of the South African Ornithologists' Union (founded in 1904) in 1905, served on its council as a representative of the Orange River Colony from 1907 onwards, and as joint vice-president for the Orange Free State from 1912 to 1916. As an enthusiastic protectionist it was largely owing to his efforts that the Korhaan, Plovers and Lapwings were added to the list of protected birds in the Free State. He also joined the Orange River Colony Philosophical Society in 1907. Johnston was hard-working and had a cheerful disposition. During World War I (1914-1918) he went to England at his own expense and placed himself at the disposal of the War Office. He was placed in the Food Protection Scheme, where he is said to have done excellent work. When the food crisis was over he returned to South Africa in 1918 on the steamer Hirano Maru, but was lost at sea when the ship was torpedoed off the Irish coast. He was married to Margaret Johnston, born Thompson, on 2 November 1908.