Henry Taipōrutu Te Mapu-o-te-rangi Mitchell was born at Ōhinemutu on 5 May 1877, the elder of two children of Te Whakarato Rangipāhere Taiehu of Ngāti Te Takinga, a hapū of Ngāti Pikiao, and Henry Walker Mitchell, a surveyor. Taipōrutu, or Tai as he was generally known, had a younger brother, James Zealand (Tīreni) Niramona. Henry Mitchell senior had arrived in Port Chalmers from Scotland in 1858. By 1867 he was engaged in survey work for the Native Land Court between the Bay of Islands and Tongariro. He then worked in the Native Land Purchase Office. As a result the family moved frequently and Tai attended several primary schools at Koutu (in Rotorua), Maketū, Waharoa and Havelock North. He received his secondary education at Wesley College, Auckland.
For about 12 months from 1893 Tai Mitchell farmed at Matatā in the eastern Bay of Plenty. During that time he showed an aptitude for survey work, which led to his joining the Lands and Survey Department as a cadet in 1894. After two years with the department he joined H. D. M. Haszard, district surveyor at Thames, and was later attached to the staff of Kenneth Rennie, roads engineer at Rotorua. He was employed in the Lands and Survey Department in 1901, qualifying as a licensed surveyor in 1902 and working mostly in the Bay of Plenty.
In 1915 Mitchell set up in private practice in Rotorua where his services were in great demand by Māori and Pākehā. He was called on by the government to undertake survey work on the island of Aitutaki in the Cook Islands during 1921, and again in 1930 when he reported to the government on the water supplies on the islands of Mangaia, Rarotonga, Ātiu and Mauke. After the latter trip he accompanied the ashes of Māui Pōmare back to New Zealand.
Mitchell accepted leadership roles in both the Māori and Pākehā communities. He served on the Rotorua County Council from 1916 to 1923, and on the Rotorua Borough Council from 1931 to 1944. His professional skills made a valuable contribution to his own people of Te Arawa as well as to people of other tribes. He became a district surveyor and director of native surveys in the Bay of Plenty; took a prominent part in the consolidation of Crown and Māori land titles in the Urewera district, acting as interpreter to Judge H. H. Carr and R. J. Knight of the Lands and Survey Department; and even planned a subdivision of the East Cape for settlement. In 1909 he surveyed some 40,000 acres of Ngāti Whakaue land at Rotorua for lease for farming. He was a member of the Waiariki District Māori Land Board from 1906 to 1913 and was appointed a member of the village committee at Ōhinemutu in 1910. In 1916 he opposed the official licensing of guides at Ōhinemutu village, on the grounds that payment for guiding would put unwanted temptations in the way of Māori, and that tourists should have free access to the settlement.
Together with Ngāti Pikiao and Ngāti Tarāwhai leaders Mitchell devoted a great deal of effort to creating scenic reserves in the Rotoiti and Rotoehu areas. As chairman of Te Arawa District Māori Council, in the early 1920s he was closely involved in organising the negotiations between government officials and Te Arawa leaders over compensation for the loss of fishing and burial rights in the Rotorua lakes district. The settlement of this dispute involved an annual payment to Te Arawa of £6,000, which was entrusted to the Arawa District Trust Board, newly established to administer the funds. Mitchell chaired the board until his death in 1944, exercising a strong influence on tribal matters.
Under Mitchell's leadership the board undertook a wide range of activities. In 1924 it purchased land at Maketū, the resting place of the Arawa canoe, for farming; the land was managed and farmed on the board's behalf. It reopened the original Kaituna River mouth, restoring pipi beds at Ngātoro; made grants to students for secondary and tertiary education; and made money available for the upkeep of marae. In 1934 the board made land at the Waihī Estuary a public reserve, naming it Bledisloe Park in honour of the departing vice-regal couple.
Mitchell's work in the 1920s was often bound up with his close friendship with Apirana Ngata. They had met about 1905 when Ngata attended a hui at Ōhinemutu seeking support for his proposals for the incorporation of tribal land; Te Arawa leaders were unco-operative and refused to help. As Ngata stepped out on to the porch at the end of the hui, Mitchell was waiting and asked him home for a cup of tea. They later worked closely together on the Rotorua lakes case, and on Ngata's development schemes.
When Ngata was appointed native minister in 1928 he was determined to push ahead with his land development policy, but he needed landowners who were prepared to commit their land for the purpose. Mitchell invited Raharuhi Pūruru, one of the leaders of Te Arawa, to discuss with Ngata the prospect of using Horohoro, an area of land that belonged to Puūuru's hapū, Ngāti Tuarā and Ngāti Kearoa. As a result Horohoro became the first land development scheme. Once the schemes had begun, Mitchell became their supervisor in the Rotorua area, working particularly on Horohoro, the Arawa–Rūātoki consolidation and the Tāheke scheme. When the Native Affairs Commission sat in 1934 the administration of the development schemes was investigated. It was found that Mitchell had been placed in an invidious position since there was a conflict of interest between his chairmanship of the Arawa District Trust Board and his duties as a departmental supervisor. He resigned from the latter position.
Tai Mitchell was recognised as an authority on matters affecting the welfare and progress of the Māori people. When the Senate of the University of New Zealand offered him an appointment to an advisory committee to consider the problems of adult education among Māori, he found himself unable to accept for fear that the time given to it might interfere with his many other responsibilities. In the Rotorua district all hui and public events of any significance were organised under his guidance. He was active in sport as a secretary of the Arawa Bowling Club, honorary surveyor to the Rotorua Racing Club, secretary of the Bay of Plenty Rugby Football Union and the Rotorua sub-union, and a member of the Māori Advisory Board of the New Zealand Rugby Football Union. In 1936 he acted as spokesman for Te Arawa in requesting the NZRFU not to schedule a game between the touring South Africans and a Māori team in 1937; he did not want Māori to be subjected to the racial abuse that had occurred on the previous tour in 1921.
Mitchell served as chairman of the Lake Okataina Scenery Board and as a member of the New Zealand Mission Trust Board and the 1940 centennial celebration committee. In 1927 he helped arrange a concert to welcome the duke and duchess of York to Rotorua, and in 1934 was one of the organisers of the tour by the duke of Gloucester. In 1928 he helped establish the Whakaue meeting house at Maketū, and he was one of the designers of St Peter's Anglican Church at Ōwhata in 1932–33, supervising its construction. He was involved in the restoration of Whakarewarewa village in 1929, and in 1933 he provided land to establish a carving workshop. In the First World War he had raised money for the Māori Soldiers Fund; during the Second World War he was active in recruiting Te Arawa soldiers. He was made a justice of the peace in 1924 and appointed a CMG in 1939.
Mitchell had married Te Aomihi Merriman (Meremana) of Ngāti Maru from Pārāwai, Thames, in 1901; she also had connections with Te Whakatōhea. According to family information, they had at least nine girls and two boys. Mitchell first saw Te Aomihi while surveying near Ātiamuri; the Mihi Bridge over the Waikato River at Ātiamuri was named after her. Mitchell was closely involved with a variety of people ranging from ministers of the Crown to dignitaries from all parts of New Zealand and overseas. He relied heavily on Te Aomihi to help provide hospitality to their many visitors, who often stayed in their home at Ōhinemutu. Whether they came for business or social reasons Te Aomihi would make them feel at home. As the children grew older they were expected to help entertain and take a greater responsibility with the work.
By the mid 1930s Mitchell had become less active; he had been suffering from diabetes, and nearly died in 1936 from complications arising from an operation for appendicitis. He died suddenly at Rotorua on his 67th birthday, 5 May 1944, survived by his wife and children. A bell shrine was erected at Ōhinemutu and dedicated to his memory, the inscription on the bell, 'Ahakoa kua mate ia e kōrero ana anō' (although dead he still speaks), indicating the esteem in which he was held and the sincere affection of the people who felt his loss deeply.