John Montagu Bain was a son of the road builder Thomas C.J. Bain* and his wife Johanna Hermina De Smidt, and a grandson of the geologist and road builder Andrew Geddes Bain*. In 1903 he donated an important collection of stone tools from various places in the Cape Colony, collected by his father and himself, to the South African Museum. Two years later J.M. Bain and Dr L.A. Peringuey*, Director of the South African Museum, found minute stone artefacts, many of them with secondary flaking, at an unnamed site in association with the bones of large mammals, including one extinct species. Bain donated further stone tools from Uniondale and Moorreesburg to the museum in 1906. These were followed by an important collection of "palaeolothic and neolithic" stone tools from the sand dunes of Fishhoek on the Cape Peninsula in 1909.
During these years Bain also collected insects. He donated various new and rare species from Darling to the South African Museum in 1905, and further specimens from near Moorressburg in 1906. In 1907 followed a collection of different insect orders made by Bain and H.M. Oakley* in "Bechuanaland" (probably British Bechuanaland, now part of the Northern Cape), including many little known species and four undiscribed ones. Bain followed this up in 1908 with a collection of coleoptera, orthoptera and diptera from British Bechuanaland, including nine new species. He also presented ants from Moorreesburg, the Vryburg district, and Morokweng (some 130 km north-west of Vryburg) during 1907-1908, and a miscellaneous collection of insects from "Bechuanaland" as well as some bird's nests and eggs in 1909.
Bain was a member of the Royal Society of South Africa in or shortly after 1908.