William Molyneux, son of William Molyneux and his wife Elizabeth, born Taston, was taken into the household of the Archbishop of York at the age of twelve and educated by the latter's sister. He developed an interest in science, though he also published a volume of poems and related material in 1853. At first he was employed in unskilled positions such as gardener and porter. However, later he turned to geology, palaeontology and archaeology, conducting excavations at Trentham, Staffordshire, and playing a role in archaeological discoveries at the coalfields of North Staffordshire. His geological and palaeontological investigations led to three papers read before the British Association for the Advancement of Science and published in its annual Reports: "On the coal straits of North Staffordshire..." (1859, with R. Garner, who taught him geology), "Remarks on fossil fish from the North Staffordshire coal fields" (1860), and "Report on the distribution of the organic remains in the North Staffordshire coal fields" (1864, 1865). Later he conducted a geological study of the Trentham region and published a paper, "On the gravel beds of Trentham Park", in the Geological Magazine (1867). When residing at Burton upon Trent he focused more on the geology and the promotion of coal mining on the Cannock Chase coalfield, some 20 km west of the town, and in 1872 reported to the British Association on "The occurrence of copper and lead ores in the bunter conglomerates of Cannock Chase". Meanwhile he had written a book on Burton-on-Trent: Its history, its waters, and its breweries (London, 1869).
Molyneux was elected a Fellow of the Geological Society of London in 1864, became a member of the British Association for the Advancement of Science in 1865, and was elected a Fellow of the Royal Historical Society in 1872. He married Caroline Amelia Pullen and they eventually had ten surviving children. One of their sons was the geologist A.J.C. Molyneux*.
Early in 1880 Molyneux left England to assist F.W. North* in a survey of the coalfields of Natal. However, a disagreement with North on the methods of investigation ended their partnership prematurely. Molyneux was then employed by the municipality of Durban to report upon the geology of the borough lands, and subsequently on possible extensions and improvements to the town's water supply. The results of this work were written up in the form of a report, Geological explorations in Natal (1880). He also wrote a Report on the coal seam and coal bearing rocks of Morewood Bay, Victoria County (1880). Next he was engaged by the government of the Cape Colony to investigate the geology of the Karoo and particularly of the Stormberg area. The results were submitted to parliament as Report on the geology of the Karroo and Stormberg, Cape Colony (G71, 1881, 42 pp), and included two locality maps and 11 sections. He concluded that no coal was to be found in the Karoo.
While in Natal Molyneux also carried out some archaeological investigations, but the only record of this work is a signed but unreferenced newspaper cutting entitled "Archaeological explorations in Natal", dated Pietermaritzburg, 13 November 1880. He mentions finding some arrow, spear and hammer heads during his excavations near Durban, also pottery near the mouth which drains the Western Vlei and elsewhere, including an almost perfect pot at a depth of 1,8 m in wind-blown sand on the Umgeni road, and shell remains on the Berea.
After returning to Natal from the Cape in 1881 he went back to England for a short period, returning with his wife and children. They settled on a farm he had bought at Inchanga, but the farming was not successful. He then undertook geological surveys in the South African Republic (Transvaal) and the Orange Free State, but found no coal. Back in Natal he made a more detailed investigation of the coalfields in the Dundee area than F.W. North had done. Realising the region's commercial possibilities he participated in forming the South African Coal and Iron Company, the first of its kind in Natal. On his way to Durban to raise money for this venture he was struck by a fever which caused his death.