Hāmiora Mangakāhia, also called Tana and later Piripi, is said to have been born in 1838 at Waikaurau, which was probably at Whangapoua Harbour on the eastern Coromandel Peninsula. His mother was Rīria Pōau (Pōnau) of Whangapoua. Because of wars between her people and Ngāpuhi, she had been born and brought up on Tūhua (Mayor Island), where she met Hāmiora's father, Piripi Te Aue Te Ikatoroa, of Ngāti Kahungunu. He had been captured, probably by Ngāti Maru in the 1820s. After intertribal fighting died down in the 1830s, Rīria took her family back to Whangapoua. Hāmiora Mangakāhia was descended from Ngāti Maru and Ngāti Tamaterā, but his most important link was to Ngāti Whanaunga. His hapū were Ngāti Hei and Ngāti Pare. He claimed land as Ngāti Huarere, the earlier inhabitants of the Coromandel Peninsula.
Mangakāhia was exposed from childhood to the new commercial world arising from contact with Europeans; his mother and her cousin Te Ngārahu arranged the early timber contracts at Whangapoua. As a youth he lived for some years with his father's cousin Wī Paekohe in Poverty Bay. In 1862 he was trained in genealogy and tradition by elders at Whareongaonga. Although he was probably a Christian from an early age and later belonged to the Mormon church, he was also a pupil of the tohunga and prophet Toiroa at Nukutaurua on the Māhia Peninsula, and considered himself bound to carry on the prophet's teaching after his death in 1867. From his brother, Mohi, who had been deprived of much of his land, Mangakāhia learned a profound and lasting distrust of Europeans. Mohi, who was a Native Land Court agent and involved in politics, was expected to stand for election to the Western Māori seat in Parliament in 1876, but died in 1875. Hāmiora Mangakāhia was the heir to both his brother's influence and his problems.
Hāmiora Mangakāhia married three times. The name of his first wife is not known. His second wife was Puriake or Pauaka of Ngāti Awa, the mother of his eldest son, Hāmiora Whakakoro Mangakāhia, and of a daughter. Father and son became estranged in later years. Mangakāhia married again, probably in the late 1880s or early 1890s. His third wife was Meri Te Tai of Te Rarawa. They had two sons, Mohi and Waipapa, and two daughters, Whangapoua Tangiora Edith and Mabel Te Aowhaitini.
Throughout his life Mangakāhia was plagued by Europeans anxious to profit from his Whangapoua lands, valued for their kauri and other timber. His battles with the Kauri Timber Company, the New Zealand Timber Company and the solicitor Frederick Earl were generally unsuccessful, resulting in the transfer of his interests to his mortgagees. Efforts from 1914 to 1916 to succeed to Wi Paekohe's interests in land around Gisborne and in Portland Island were also unsuccessful. At the time of his death, of all the thousands of acres once controlled by his family, Mangakāhia retained only one major block.
For a time Mangakāhia felt the only solution to Māori economic problems was to abandon Māori customs of hospitality, as the proceeds of land sales were being consumed by chiefly extravagance. His own unhappy experiences with land matters made him a shrewd counsel in the Native Land Court, where he not only represented his own kin, but was invited to conduct cases as far afield as Cambridge, Alexandra (Pirongia) and Napier. He also became an assessor of the court, practising at hearings across the country, including Maketū, Mōkau, and some of the various hearings into the Horowhenua block. In 1891 Mangakāhia was one of few Māori whose views were sought in Auckland, and again at Waipāwa, by the Māori Land Laws Commission. He advocated setting up a tribunal with powers to settle all outstanding land disputes between Māori and Europeans, and the repeal of all legislation in connection with the Native Land Court.
Hāmiora Mangakāhia's greatest political achievement was his contribution to Te Kotahitanga, the Māori parliament movement. He was at the Bay of Islands in 1889 when Ngāpuhi formally initiated an attempt to forge the political union of the North and South Island Māori tribes, and was among the first signatories of the movement's deeds of union. Mangakāhia was selected by Heta Te Haara to explain the purpose of the union to delegates at the preliminary session held at the meeting house Te Tiriti o Waitangi at the Bay of Islands in April 1892. Mangakāhia explained that the movement hoped to establish a Māori government to control Māori land and other matters. Such an institution was authorised by section 71 of the 1852 constitution, he claimed, and it would be the executive arm of a national Māori union under the authority of the Treaty of Waitangi. The treaty established the Queen's authority over the whole country but also established that Māori alone had authority over their lands. Under Māori custom, Mangakāhia pointed out, one authority would never encroach on another. He also stated that if the movement provoked serious trouble between Māori and Europeans it would have to be abandoned.
On 20 April 1892 Mangakāhia was appointed to a committee to reconsider the movement's aims. Its report advocated the abolition of the Native Land Court and its Māori assessors, and all related legislation. Māori committees should be established to consider lands still under Māori title; land court sessions, with the exception of rehearings, were to be boycotted. By the end of the preliminary session Mangakāhia, working alone, prepared all the documents for the first sitting of the Māori parliament, to be held at Waipatu in June 1892. On 17 June, nominated by Hēnare Tomoana and Te Keepa Te Rangihiwinui, Mangakāhia was elected premier of the Great Council (the lower or elected house). Four days later he presented a bill requesting that a petition be sent to the colonial parliament asking that all legislation on Māori land and people cease, and that Māori be empowered to make their own laws.
Mangakāhia was not re-elected for the second sitting of the Māori parliament in 1893, but continued to be called on for advice. His plain speaking may have caused offence, for in April the Council of Paramount Chiefs (the upper house) was considering his expulsion from the movement, but on 1 May he was sworn in as a member of the Great Council. He told the house that he was willing to stand for the colonial parliament, and that he had given up an annual income of £500, a sum which included his salary as Native Land Court assessor, in order to devote himself to the work of Te Kotahitanga.
Mangakāhia was re-elected premier at the fourth session, held at Rotorua in 1895. This session was noted for the obstruction by Hēnare Tomoana, who styled himself leader of the opposition and attempted to force Mangakāhia's resignation on the grounds that Te Kotahitanga funds had been mismanaged. Mangakāhia, with support from many other chiefs, was able to show that the movement's financial problems had developed under the premiership of his successor in 1893, Te Whatahoro Jury.
Apart from personal ambitions, the issues between Mangakāhia and Tomoana reflected a growing split in the Kotahitanga movement, which was never to be resolved. Mangakāhia represented those who refused to accept a moderate line acceptable to the colony's government. This split grew out of two bills asking for separate powers for Māori, presented by Hōri Kerei Taiaroa and Hōne Heke. Taiaroa's bill sought power of assembly for the Kotahitanga parliament which would pass bills that the governor would sign into law. Mangakāhia favoured Heke's bill, which asked for separate Māori institutions and excluded the governor.
Although still officially premier in 1897, Mangakāhia was prevented from attending either the April or October sessions by the death on 17 April of his 26-year-old daughter from his second marriage, and by a severe attack of rheumatism. In March 1898, still premier, he claimed in Te Puke ki Hikurangi that between 13 March 1889 and 6 October 1897 the Kotahitanga deeds had been signed by 37,000 Māori. He warned the people that the colonial government had mortgaged the country to English interests, and advised them to hold fast to the Treaty of Waitangi, to the 71st clause of the 1852 constitution, and to the Kotahitanga deeds they had signed.
The split within Te Kotahitanga became more obvious after the moderate party, led by Tomoana and Tamahau Mahupuku, succeeded in having the 1897 and 1898 sittings of the parliament transferred to Pāpāwai. The issue now became the Native Lands Settlement and Administration Bill. It was promoted at Pāpāwai by the premier, Richard Seddon, who promised that if Parliament passed this bill the government would purchase no more Māori land, and would not permit Europeans to do so. Mangakāhia's party regarded the bill's provisions as paternalistic, and he explained their objections at meetings with Seddon in Wellington in July 1898. Land given up to the proposed boards would effectively be controlled by the government, the boards' expenses would swallow up the revenue, and all that would be left to the owners of the land would be their names on a piece of paper. He and others collected 9,775 signatures to their petitions declaring their opposition to the bill.
Mangakāhia spent most of his later years at Te Pungapunga, Whangapoua, attempting to settle his private affairs. In 1907 he attended a meeting at Waahi, Huntly, organised to discuss Māori grievances arising from breaches of the Treaty of Waitangi, and to form a new union under the authority of the treaty and the 1852 constitution. The resurrected Kotahitanga would enable a united Māori appeal to the imperial government. Mangakāhia reminded the meeting of earlier efforts, and approved of the plan to send a deputation to England, but it failed to go ahead as he had hoped.
Mangakāhia was at Pāpāwai again in 1911, but this would have been one of his last excursions; he was in a wheelchair in his last years. According to family information, he died on 4 June 1918, and was buried on a knoll above his homestead at Whangapoua.