William Henry Addison studied medicine at King's College, London, and was admitted as a member of the Royal College of Surgeons of England (MRCS) in 1842. Continuing his studies he qualified as a licentiate in midwifery at Dublin in 1844, and as Doctor of Medicine (MD) at Edinburgh in 1845. In May 1849 he arrived in Natal and from early in 1850 served as district surgeon of Pietermaritzburg, while simultaneously developing a private practise. In 1854 he collected reptillian fossil bones near Mooi River, which were described by the government geologist P.C. Sutherland* in 1855 as similar to those sent to the British Museum by A.G. Bain.
Resigning as district surgeon in 1853, Addington bought the farm Rietvallei near Howick, but in 1858 moved to a coastal farm on the banks of the Umvoti River near present day Stanger. There he experimented with the cultivation of coffee, cotton, arrowroot, indigo, and sugar cane, the latter with much success. He also proposed to produce natural fibers suitable for rope manufacture from wild hemp and wild banana plants. Meanwhile his services to medicine continued, first as district surgeon of the Tugela division, and from 1868 to 1888 as district surgeon of Durban. In addition he served as health officer to the port of Durban from 1868 to 1883, and as surgeon at the government hospital (later Addington Hospital). During 1871-1872 he was the founding president of the Durban Medico-Chirurgical Society, which had eight founding members and survived to about 1884. Addison served as treasurer during all this time. His third son, also named William Henry (1852-1939), studied medicine in London and joined his father's Durban practice in 1880. In 1888 the elder Addison retired to his farm Rietvallei, where he remained for the rest of his life. He was the progenitor of one of the leading families of Natal.