John Buchanan was awarded the degree Master of Arts (MA) by the University of Glasgow, and was ordained at Bothwell, Scotland, in 1844. In 1846 he was one of the deputies of the Free Church of Scotland to Canada. He emigrated to Natal in 1861 for health reasons. There he set up the Presbyterian church in Durban and Pietermaritzburg and visited variuos mission stations. During his travels he collected plants in many places, such as between Durban and Pietermaritzburh, at Mapumulo, around Greytown, and as far north as Newcastle. He was interested specifically in ferns and grasses. In 1867 he and Mark J. McKen* sent some 135 specimens of ferns to the herbarium at Kew Gardens, and Buchanan provided most of the locality records for McKen's The ferns of Natal. In 1868 and 1869 he served on the management committee of the Natal Agricultural and Horticultural Society, which managed the Natal Botanic Gardens in Durban, and was vice-president the next year.
In 1874 Buchanan resigned his ministry when his health failed again. During his illness he compiled a Revised list of the ferns of Natal, based mainly on sources such as Pappe* and Rawson's* Synopsis filicum Africae australis and Lady Barkly's* Revised list of the ferns of South Africa, as well as an examination of the collections of, among others, Sir Henry Barkly* and Rawson. The list first appeared in the form of articles written for the Natal Colonist, and was reprinted as a 30 page pamphlet in Durban in 1875. During this year he also revised and expanded the collection of the ferns of Natal that he had previously presented to the herbarium of the Natal Botanic Gardens in Durban. Hundreds of his specimens are still in the Natal Herbarium.
Buchanan then left Natal and travelled through Lesotho to the Orange Free State. At Leribe, Lesotho, he collected mainly grasses, before moving on to Winburg and Kimberley. He then went to Bloemfontein, and travelled via Burgersdorp and Queenstown to the Lovedale Mission, near Alice, where he acted as locum tenens during 1876-1877. Here he read an address to the members of the Lovedale Literary Society, which was printed by the Lovedale Missionary Press in 1877. He also collected plants in the Amatole Mountains and assisted J.M. Wood* with The classification of ferns (1879). Leaving South Africa at this time, he took charge of the Presbyterian congregation on the island of Madeira from 1878 to 1880, before retiring to Largs, Scotland. However, in 1880 he was still on the voters role for Durban as a property owner.
Buchanan sent more than a thousand plant specimens to Kew Gardens between 1867 and 1883, and there are also specimens from him in the Albany Museum (Grahamstown) and in the herbarium of the South African Museum. However, the localities on his lables were sometimes wrong, for example, Kalahari grasses were labelled as from Lesotho. Several species were named in his honour.