Johnston was one of the founders of the Berwickshire Naturalists' Club and became its first president.
Johnston was educated first at Kelso, then at Berwick grammar school, and finally at the University of Edinburgh.
He was apprenticed to John Abercrombie, and in 1817, qualifying as a member of the Royal College of Surgeons of Edinburgh, he went to London.
He was one of the founders of the Ray Society and of the Berwickshire Naturalists' Club.
To it, the Edinburgh Philosophical Journal, Loudon's Magazine of Natural History, the Transactions of the Natural History Society of Newcastle, and to the Proceedings of the Berwickshire Naturalists' Club, he contributed 90 papers.