Johnston was born at Laputa, near the townland of Clyhore, between Ballyshannon and Belleek in the south of County Donegal in Ulster. He was the son of Robert Johnston, who served as a Q.C. in County Donegal. He was educated at King William's College on the Isle of Man.
Rugby international
Ireland
In 1893, Johnston made 2 appearances for Ireland. He made his international debut on 4 February 1893 in a 4–0 defeat against England at Lansdowne Road. Then on 11 March he played in a 2–0 defeat against Wales at Stradey Park . Two of his brothers were also Ireland internationals
When the British Lions tour ended, Johnston, together with Thomas Crean, decided to stay on in South Africa. He played rugby for Transvaal and captained them in the Currie Cup. In 1899, at the start of the Second Boer War, again with Crean, he enlisted in the Imperial Light Horse and Johnston subsequently reached the rank of major. Johnston was already an experienced soldier, having previously served with the Royal Inniskilling Fusiliers between 1890 and 1894. He served in South Africa between 1899 and 1901 and was dangerously wounded during the Siege of Ladysmith. In October 1899, according to the Irish Times, the Imperial Light Horse particularly distinguished themselves in the Battle of Elandslaagte and it was during this engagement that Johnston, while still a captain, won his VC. The joint citation for him and Captain Charles Herbert Mullins reads: Johnston was badly wounded and was nursed back to health by Crean. He travelled to London in early 1901, and both he and Mullins received the VC from King Edward during an investiture at Marlborough House 25 July 1901. Johnston was awarded the Queen's South Africa Medal with clasps Elandslaagte and Defence of Ladysmith and the King's South Africa Medal with clasps South Africa 1901 and South Africa 1902 for his service in South Africa. The location of his medals is unknown. Mullins was the brother of Cuth Mullins who had earlier been one of Johnston's British Lions team-mates. In 1902 he was Commandant at a concentration camp at Middelburg, Mpumalanga and in 1903 he was a District Commissioner in the Eastern Transvaal.
Later years
In 1911 Johnston returned to Ireland and joined the General Prisons Board for Ireland. He was Commandant of the P.O.W. camp at Oldcastle 1914–15, and was appointed Governor of His Majesty's Convict Prison at Maryborough in 1915, before returning to Oldcastle in 1916. In 1918 he was appointed a resident magistrate. He later settled in Kilkenny, where he died. Conflicting sources give his date of death as either 1950 or 1970. He was buried in St. Mary's Churchyard, Inistioge, County Kilkenny.