Edwin Mitchelson was the son of Alexander Mitchelson and his wife, Johanna Fitzpatrick, who arrived at Kaipara Harbour, New Zealand, from Sydney, Australia, on the Hannah Watson in April 1840. They settled at Maungaturoto, but when Alexander Mitchelson failed to find employment they moved to Auckland, where he worked in a warehouse. Edwin was born in a cottage in Queen Street, probably on 7 April 1846.
Edwin attended St Matthew's parish school and Mr Paterson's private school. In the late 1860s, after serving his apprenticeship as a carpenter, he went to Northern Wairoa, Northland, an important centre of the timber and gum export trade, where he worked as a carpenter and builder. There, on 12 September 1871, he married Sarah Wilson, daughter of John Wilson, a pioneer farmer in the district; they were to have two daughters and two sons.
In 1871 Mitchelson joined the firm of J. M. Dargaville and Company, timber and kauri gum merchants. He was soon promoted to manager of their store and trading concerns. The area's first post office was located in this store and Mitchelson was postmaster from 1871 to 1880. In 1876, when Joseph Dargaville sold the timber interests, Mitchelson took over the other enterprises. With his brothers Richard and John he founded the firm of E. Mitchelson and Company, which later extended its activities to include the timber trade. He was also involved in founding the shipbuilding industry on the Kaihu River. He owned several ships, including the fine schooner Huia, trading regularly between Kaipara Harbour and Lyttelton. Mitchelson was one of the original promoters of the Kaihu Valley railway.
When Mitchelson moved to Auckland in 1881 he established a branch of E. Mitchelson and Company, with offices and a gum warehouse in Little Queen Street. From the 1890s the output from the sawmills he had built at Aoroa and elsewhere in the north was prodigious. In December 1898 a new company, Mitchelson Timber Company, was formed and the head office transferred from Dargaville to Auckland. At the peak of sawn timber production in 1907 the company held cutting rights to kauri on Crown lands in Hobson, Hokianga and Whangarei counties.
In 1876, as one of the most influential merchants in the region, Mitchelson was elected to the first Hobson County Council. In 1881 he entered Parliament as member for the Marsden electorate. A staunch supporter of Sir Harry Atkinson, he was appointed minister for public works in the Atkinson government of 1883–84 and the short-lived ministry of August 1884. In 1887, when Atkinson gained power again, Mitchelson, now member for Eden, became minister for public works; he held this portfolio until his appointment as postmaster general in 1889. He supported the premier's modest protectionism, believed that the state should play a role in the development of the nation's resources and infrastructure, and was much respected for his liberal views and able administration.
Mitchelson also served as minister for native affairs from 1887 to early 1891. With a view to the eventual abolition of the Native Department, he sharply reduced its expenditure and transferred some of its functions to other departments. His Native Land Act of 1888 restored direct purchase of Maori lands and removed safeguards against the alienation of reserves. It was passed despite the bitter opposition of the Maori parliamentarians. In 1890, because of Atkinson's ill health, he acted as premier, colonial treasurer and commissioner of trade and customs for the remainder of the term of office. He represented Eden for two further terms, during which he spoke and voted against the governments of John Ballance and Richard Seddon, but was defeated in 1896 for City of Auckland.
For some years after his defeat, Mitchelson's business commitments required his close attention. In addition, he continued his campaign for the extension of the railway north of Helensville. By 1903 he had found time to turn his attention to public affairs. From 1903 to 1906 he was chairman of the Remuera Road Board, and he was mayor of Auckland from 1903 to 1905. As mayor he was responsible for planning a more efficient and extended water supply, for developing parks and recreation grounds, and for persuading the council to offer a building site to the Leys Institute, which he officially opened on 29 March 1905.
Mitchelson was a founder and long-serving chairman of the Diocesan High School for Girls. He was also associated with the Auckland University College council, the Auckland Grammar School, the Auckland Harbour Board, the Auckland Savings Bank, and the Auckland Institute and Museum. In 1913 he presented his complete collection of 400 specimens of kauri gum to the museum. He was made a knight of grace of St John of Jerusalem for his services to the St John Ambulance Association. He was made a KCMG and appointed to the Legislative Council in 1920.
Mitchelson had a long association with horse-racing. He joined the syndicate which had in 1879 founded Auckland's first stud farm at Glen Orchard. He was a member of the Auckland Racing Club (1883–1932), and served as president for three terms and then continuously from 1905 to 1932. He owned several valuable and well-known racehorses. The creation and development of the magnificent Ellerslie racecourse gardens was largely due to his enthusiasm and expertise; they are his most outstanding legacy to the people of Auckland. In 1929 a bronze bust of Mitchelson was erected in the gardens.
Edwin Mitchelson died at Auckland on 11 April 1934; he was survived by his wife, a son and a daughter. Numerous tributes testified to the high esteem and affection in which Mitchelson was held. His success as a businessman in Northland and Auckland, his record of service in local affairs, and his contribution to the development of Auckland all testify to his energy and abilities.