John Buchanan is said to have been born on 13 October 1819, at Levenside (Strathleven), Dunbartonshire, Scotland. He was the eldest of four children of William Buchanan and his wife, Nancy Somerville. He was educated at the local parish school and mechanics' institute and apprenticed as a pattern designer at a calico printworks in the Vale of Leven. Later he was foreman of the drawing shop of Henry Monteith and Company, Barrowfield, and lived at Busby, Glasgow, where he devoted his leisure to botany. He never married. In July 1851 he emigrated to New Zealand on the barque Columbus, arriving at Port Chalmers in February 1852.
After a brief sojourn on the Victorian goldfields, Buchanan returned to New Zealand to farm his 10 acre section in North East Valley, near Dunedin. He was employed by Alexander Garvie as an assistant on the provincial reconnaissance surveys between 1856 and 1859 and in March 1858 was the first to find gold at the Tuapeka and Clutha rivers. In 1861 and 1862 he went prospecting in the Tuapeka and Manuherikia regions. During his journeys Buchanan collected plants to send home to his friend Dr John Ross, a medical practitioner and amateur botanist, in Busby. Ross recommended Buchanan to Joseph Dalton Hooker as a knowledgeable botanist to serve on James Hector's forthcoming geological survey of Otago. In the spring of 1862 Hector engaged Buchanan first privately and then officially as botanist and draughtsman for both his expeditions of 1863, when he explored the West Coast by land, and also by sea in the schooner Matilda Hayes.
Under Hector's scientific direction Buchanan had, by the end of 1864, produced his finest work, including maps, botanical drawings and watercolour panoramas of the western lakes and sounds. Of this last group, 'Milford Sound looking north-west from Freshwater Basin' is the best known. He had also written 'Sketch of the botany of Otago', published in the Transactions and Proceedings of the New Zealand Institute in 1868, and had accumulated large plant collections. All were displayed to illustrate the work of the Geological Survey of Otago at the New Zealand Exhibition of 1865. Later that year Hector, as director of the newly constituted Geological Survey and Colonial Museum in Wellington, secured appointments for all his Dunedin staff in their former capacities. Buchanan came to live in Wellington, and lodged at Pipitea Street, Thorndon, for the next 20 years.
Botanical and geological trips continued to take Buchanan to all parts of the country, and were documented in field-books and many landscape and natural history drawings. Sketches, paintings, reports and personal papers are now preserved in libraries and museums in Wellington, Auckland, Dunedin and Sydney. Buchanan published 36 scientific papers as well as the three volume, illustrated folio work The indigenous grasses of New Zealand (1878, 1879, 1880).
As the Colonial Museum became more important to the scientific community, so Buchanan's responsibilities grew. Among his professional associates were W. B. D. Mantell, W. T. L. Travers, H. H. Travers, A. McKay, T. Kirk and W. Skey. He was a foundation member of the New Zealand Institute and lithographed the illustrations for the Transactions from 1867 until 1885. In 1876 he became a fellow of the Linnean Society, and on his retirement from the Colonial Museum in June 1885 was made a life member of the Wellington Philosophical Society. A portrait of him at this time shows that he was slim, with a bright complexion and deep-set blue eyes. In 1885 Buchanan returned to his home in North East Valley, Dunedin. He died on 18 October 1898.