William Gill qualified as a medical practitioner by completing a five year apprenticeship (1810-1815) with Thomas Peck of Wellingborough, Northamptonshire, and then completing his studies at the University of Glasgow. In 1818 he was admitted as a member of the Royal College of Surgeons of England (MRCS) and a licentiate of the Apothecaries Company. Meanwhile he spent a year touring continental Europe to further his studies in botany, which remained a life-long interest. He came to the Cape Colony in June 1818 and was licensed to practice there as a surgeon, accoucheur and apothecary in July or August. After a few years as district surgeon at Caledon he settled in the Zwartland where Malmesbury later developed, in September 1822. Here he earned praise for his zeal in fighting a typhoid epidemic.
In 1826 Gill set off northwards to hunt, botanise, and collect birds and other zoological specimens. He reached Kuruman and met A.G. Bain* there. However, he lost valuable collections when his waggons were destroyed, one by fire and another by elephants. He returned to Cape Town in 1827. At some time during the next three years he collected shell and plant fossils east of Uitenhage on the banks of the Sondagsrivier (Grisbrook, 1830).
In Jule 1829 he was appointed district surgeon at Somerset East, having been recommended for the post by his friend, Rev. George Thom*. He also started a practice and as a kind and generous man endeared himself to the community. His travels in search of natural history specimens were now over, but he continued to search for rare orchids. He sold about 400 botanical specimens to Thom, who sent them to W.J. Hooker at Glasgow. They eventually ended up in the Herbarium at Kew Gardens. The plant species Myrsine gillianus, Aspalathus gillii, and Erica gillii were named after him.
The district surgeoncy of Somerset East was abolished in 1845, at which time Gill declined the position of resident surgeon on Robben Island and was allowed an annual pension of 50 pounds sterling. Though he did not become rich from his medical practice he acquired several farms and successfully bred merino sheep. In 1832 he assisted in founding the Somerset Reading Society and in 1856 was a member of the Literary, Scientific and Medical Society of Grahamstown. He died of dropsy at the age of 68, leaving an amount of 23 000 pounds sterling to "found and maintain an institution of higher education in the Eastern Province". This bequest was used to establish a high school and university training centre for boys in Somerset East, named Gill College. It opened in 1869, but lost its university college status in 1903 to become a high school only. Gill was not married.