William Prosser, Fellow of the Geological Society of London (FGS), published a single paper on his geological work in England: "The fossiliferous character of the millstone-grit at Sweeney, near Oswestry, Shropshire" (Geological Magazine, 1865). Later he came to the Cape Colony to study its geology, remaining from 1877 to 1879. His first paper resulting from this work, "The water supply of the Karoo", appeared in the Cape Monthly Magazine (Series 2, Vol. 15, pp. 35-40) in July 1877. It was soon followed by a semi-popular account entitled "The Karoo, No. II" (Vol. 15, pp. 99-103). At the second ordinary meeting of the South African Philosophical Society, on 31 October 1877, Prosser was elected a member. However, he was no longer listed as a member in December 1879, by which time he had presumably left the colony. During his brief membership he presented several papers before the society: "On the Devonian and ash beds of the Karroo and Gouph" (31 October 1877; Transactions, Vol. 1(1), pp. 13-35), "On the limestones of some parts of the Cape Colony" (30 January 1878; Vol. 1(2), pp. 47-53), "On the granite and gneiss of the colony" (27 February 1878; Vol. 1(2), pp. 93-100), and "On the coal fields of the colony" (24 April 1878; not published). He intended also to compile a handbook on the geology of the Cape, but no such work is known.
Prosser collected fossils in the Hex River Pass and between the top of the pass and Hottentotskloof to the north, but did not pay much attention to the stratigraphy. The beds that were later described as Dwyka tillite he regarded as altered beds of ash and noticed that their deposition marked a period of great change in the local flora and fauna. He described the limestones that he had observed between Cape Town and the Karoo, particularly the fossiliferous limestone exposed in the railway tunnel in the Hex River Pass. His account of the granite, gneiss and slates near Cape Town was less clear and accurate than earlier accounts by Dr Clarke Abel*, Captain Basil Hall*, and Charles Darwin*.
Prosser must have had a good background in palaeontology, for during 1877 he identified the lower Karoo fossils in the collection of the South African Museum, Cape Town. He was interested also in the culture of the San, for at a meeting of the South African Philosophical Society on 29 May 1878 he formally proposed: "That it is desirable to form a section of the South African Philosophical Society, to be entitled the Ethnological Section, which, while paying attention to Ethnology generally, will devote itself specially to the work of the preservation of Bushman drawings and other Bushman remains." (Transactions, Vol. 1, p. vii). Nothing seems to have come of this proposal.
It appears that Prosser returned to southern Africa a few years after his departure from the Cape Colony, for in July 1883 one William Prosser, presumably the same person, applied for a concession from the government of the South African Republic (Transvaal) to smelt silver from lead ore in the Pretoria region. The concession was granted in December that year. In May 1884 he inspected gold fields in the Soutpansberg and two months later informed the government that he had found gold both there and in the Waterberg. Soon thereafter he offered his services to the government as a prospector. At some time between 1885 and 1890 he settled in or near Barberton, where he died. He was survived by his son, William Alexander Prosser.