Johnston was one of the founders of the Berwickshire Naturalists' Club and became its first president. His books include The Flora of Berwick-upon-Tweed, History of British Zoophytes, and History of British Sponges and Lithophytes.
In 1818 Johnston began practice at Berwick-on-Tweed, where he remained. In 1819 he graduated M.D. of Edinburgh, and in 1824 became F.R.C.S.E. He was thrice mayor of Berwick, and became LL.D. of Aberdeen. He retired from practice in 1853, and died at Berwick on 30 July 1855. He was one of the founders of the Ray Society and of the Berwickshire Naturalists' Club.
Flora of Berwick-upon-Tweed, vol. i., 1829, vol. ii., dealing with cryptogams, 1831.
Address to the Inhabitants of Berwick on Cholera, 1832.
History of British Zoophytes, from the Transactions of the Newcastle Natural History Society, 1838; 2nd edit. 1847.
The Molluscous Animals, in the English edition of Georges Cuvier's Animal Kingdom, 1840.
The History of British Sponges and Lithophytes, 1842.
Introduction to Conchology, 1850, reprinted from John Claudius Loudon's Magazine, in which it had the title Natural History of Molluscous Animals.
Terra Lindisfarnensis: the Natural History of the Eastern Borders, vol. i., Botany, with the popular names and uses of the plants, and the customs and beliefs which have been associated with them, 1853 (no more published).
Catalogue of the British non-parasitical Worms in the Collection of the British Museum, completed just before Johnston's death, but not published by the trustees until 1865.
Catalogus animalium et plantarum quæ in insula Lindisfarnensi visa sunt mense Maio, 1854, printed in the Proceedings of the Berwickshire Naturalists' Club, 1873, vii. 46.