John Morrison Leslie, son of the apothecary John Leslie and his wife Anna Elizabeth Pannell, qualified as a chemist and druggist in Britain in 1873 and was registered to practice in the Cape Colony in October 1874. At some time during the next eight years he worked as a dispenser at Grey Hospital in King William's Town. He resided in Port Elizabeth from 1882 or earlier and in 1903 was still listed as a chemist and druggist in the town, although he became insolvent in 1900. In 1890, at the annual meeting of the South African Pharmaceutical Association (established in 1885), held in Port Elizabeth that year, he urged with enthusiasm that the inactive association should continue to exist. At some time during his career he assisted with the development of a dry-earth closet system to dispose of human waste. During the earlier part of his residence in Port Elizabeth he was an active naturalist, and by 1888 had been elected a fellow of the Zoological Society of London.
Leslie was an early member of the Port Elizabeth Naturalists' Society (1882-1884), which was reconstituted as the Eastern Province Naturalists' Society in 1884. During its first year (1882) he delivered a lecture on mineralogy. However, by 1884 his main field of interest was herpetology. At a meeting of the society on 19 September that year he exhibited some Cape snakes, and indicated that he was communicating with naturalists in England with a view to compiling a book on South African snakes. He therefore appealed to the public to send him specimens. At a conversazione held by the society in December 1884 he exhibited cases of snakes and birds, as well as a small herbarium. His herpetological activities were recognised also outside Port Elizabeth, for in May 1885 Miss Mary Glanville* informed members of the Grahamstown Natural History Society that "Mr J.W. Leslie" of Port Elizabeth was cataloguing and describing Cape snakes. In August 1885 he presented the first of a series of papers on South African snakes to the Eastern Province Naturalists' Society, dealing with the egg-eating snake Dasypeltis scabra. During that year he also presented snakes and tortoises to the Port Elizabeth Museum. In July 1888 he delivered a more general lecture on the classification and identification of South African snakes. The next year he read a paper on the life history of the common platanna, Xenopus laevis. His observations on this species were published as "Notes on the habits and oviposition of Xenopus laevis" in the Proceedings of the Zoological Society of London (1890).
Leslie was the leading scientist of the Eastern Province Naturalists' Society for several years. At the annual meeting in January 1887 he reported upon its work in mineralogy as well as on reptiles. In January 1891 he exhibited a splendid collection of archaeological remains from kitchen middens around Algoa Bay, and was considered to be almost wholly responsible for the society's scientific activities during the past year. In February 1892 he gave a lecture on the comparative anatomy of the skulls of reptiles, birds and mammals, and was praised for the depth of his scientific studies. He was also active as an office bearer of the society, serving as honorary treasurer in 1886, president in 1887, 1891 and 1892, vice-president and representative of the society on the committee of the Port Elizabeth Museum in 1888, and a member of the society's management committee in 1889 and 1890.
Though his scientific activities were confined to Port Elizabeth, Leslie also presented some specimens to museums elsewhere. For example, in 1894 he donated a human skull and a few stone artefacts from a midden at the mouth of the Zwartkops River (in Port Elizabeth) to the Albany Museum, Grahamstown. The next year he presented various South African minerals and fossils to the South African Museum in Cape Town. At this time he appears to have ended most of his scientific activities, though the Port Elizabeth Museum received an unidentified locust from him in 1899.