William Carmichael McIntosh was educated at Madras College, St Andrews, and at the University of St Andrews. Proceeding to Edinburgh he studied medicine there, qualifying (with distinction and a gold medal) in 1860 as Doctor of Medicine (MD) specialised in mental diseases. His thesis, reflecting a life-long interest in natural history, dealt with Observations and experiments on the Carcinus maenas (shore crab) and was published in London in 1861. From 1863 to 1882 he was medical superintendent of the Perth District Asylum at Murthly, just north of Perth. During this period he published many zoological papers - a number of them were reprinted in The marine invertebrates and fishes of St Andrews (Edinburgh, 1875). He was mainly a descriptive zoologist and his most important work was a classic account of the Nemertines (ribbon-worms) and the Polychaete worms, Monograph of the British marine annelids, issued in 9 parts from 1873 to 1923. He also described the Polychaete worms collected by the Challenger expedition of 1873-1876, in two volumes published in 1885.
In 1882 McIntosh was appointed professor of zoology at the University of St Andrews, where he remained until his retirement in 1917 at the age of 78. Soon after his appointment he started investigations on the Scottish sea fisheries and set up the country's first marine laboratory at St Andrews. His work there made him one of the pioneers in fishing research. He published numerous notes and papers, most in the Annals and Magazine of Natural History, on fishes and fisheries, as well as books on The life histories of the British marine food fishes (with A.T. Masterman, London, 1897) and The resources of the sea... (London, 1899).
The government biologist of the Cape Colony, J.D.F. Gilchrist*, sent McIntosh the marine worms collected during the marine biological survey of South African coastal waters around the turn of the twentieth century. He reported on these in "Marine Annelids (Polychaeta) of South Africa" in Marine investigations in South Africa (1905, Vol. 3, pp. 17-56, 57-92). Almost 20 years later Gilchrist sent him further marine Polychaetes from South Africa, which he described in two notes in the Reports of the South African Fisheries and Biological Survey (1922, No. 3, and 1923/5, No. 4).
McIntosh never married. In 1869 he was elected a Fellow of the Royal Society of Edinburgh, and in 1877 a Fellow of the Royal Society of London. Honorary degrees were conferred on him by the Universities of St Andrews, Edinburgh, Oxford, and Durham.