Hōne Mohi Tāwhai was the son of Mohi Tāwhai and his wife, Rāwinia Hineikoaia (also known as Hārata). His father belonged to Te Māhurehure, a hapū of the Ngāpuhi confederation, and the family was also connected with Te Whānau Puku. Te Māhurehure held the territory of Waimā in Hokianga, and it was probably there that Hōne Mohi Tāwhai was born in 1827 or 1828. He was about 12 years of age when his father signed the Treaty of Waitangi at Hokianga on 12 February 1840. Little is known of his early life, but it is likely that he attended a local mission school and was associated with the Wesleyan missionaries in the area. His name, Hōne Mohi (John Moses), is probably a baptismal name.
Tāwhai assumed the responsibilities of leadership early in life. When the official rūnanga at Waimate North (established under Sir George Grey's rūnanga system) became defunct in 1865, he kept an informal one functioning in Hokianga. He was appointed as an assessor to the Native Land Court in the north and influenced the people at Waimate North to co-operate peacefully with the court. He became highly regarded, and in 1867 was recommended by the resident magistrate E. M. Williams to the government for commendation for negotiating peace during an inter-tribal conflict that had led to blows. When quarrels again broke out over land at Ōtaua in 1882, the resident magistrate Spencer von Stürmer acknowledged the part played by Tāwhai in preventing bloodshed.
By 1876, however, Tāwhai had become disillusioned with the Native Land Court. The expense of getting a Crown grant, including survey and court costs, was seen as an unnecessary financial imposition. In June, in a letter to the council of Tūhourangi at Rotorua, he indicated that Ngāpuhi wanted the court abolished and the law changed so that land which had not been put through the court would remain under ancestral title. Nevertheless, he represented his hapū before the court, and conducted their cases for major claims such as the Waimā block in 1885 and 1886.
Tāwhai was the member of the House of Representatives for Northern Māori from 1879 to 1884. In January 1880 he was appointed to sit on the West Coast Royal Commission to inquire into Māori claims against land confiscations in Taranaki and the detention of prisoners who resisted the government survey. Initially, he did not want to take his place, but he was persuaded by the minister for native affairs on the grounds that he was impartial, not having been involved in the wars.
Before the commission sat, Tāwhai voted against the Māori Prisoners Detention Bill of 1880 because it had not been printed in Māori and was being rushed through the House: he refused to support 'blindfold' a measure that was hurriedly presented. He also criticised the shipping of Māori prisoners from Taranaki to gaols in Dunedin and Hokitika as a move to get rid of them; he felt they would perish in the colder climate of the South Island. Tāwhai then declined to sit on the commission with William Fox and Francis Dillon Bell, because the former was implicated in the Taranaki confiscations and the latter in the purchase at Waitara.
Although he had little formal education, Tāwhai was naturally talented and made a strong contribution to parliamentary debates on a wide range of issues. He was independent-minded and claimed the right to vote as he pleased. He was particularly critical of the government's borrowing from Britain to pay for a war he regarded as aimed at destroying the people at Waitara. He opposed the introduction of legislation designed to alienate Māori land, believing that the proceeds of land sales would be used to help defray interest on the loans. He also criticised the expenditure of more than £20 million borrowed for public works in the 1860s and 1870s, objecting to the imposition of a property tax on his constituents to pay for the loans when none of the money had benefited the north. He claimed that other districts received benefits in the form of railways and public works but Hokianga had received nothing. He warned that if the government persisted in taxing Māori land it would need an army of 24,000 to enforce the taxes. Tāwhai voted against the Crown and Native Lands Rating Bill 1880 on the grounds that taxing Māori land was another step towards its alienation.
In 1881 and 1882 Tāwhai and Hēnare Tomoana were responsible for the Native Committees Empowering Bill; it provided for committees which would replace the Native Land Court and determine titles more rapidly and honestly. The government sponsored a modified version the following year, but it was not a success. In 1881 Tāwhai accused the government of betraying its duty to protect the Māori and ward off such evils as might threaten them; instead, it had oppressed them. Because the Legislative Council had produced bills which he approved of, such as the Native Lands Frauds Prevention Bill and the Native Succession Bill, he thought that the House of Representatives should be abolished and the council retained. When suggestions were made in the debate on the Representation Bill that the Māori seats should be abolished, Tāwhai defended them. He argued that the right to representation emanated from Queen Victoria through the Treaty of Waitangi; if members could show him an act or law which had abolished the treaty, then he would say no more.
As an advocate of Māori rights, Tāwhai argued that laws not made in accordance with the Treaty of Waitangi were the cause of ill-feeling between Māori and Pākehā. He believed that Māori aspirations could be achieved by working through Parliament. Accordingly, in the early 1880s he remained aloof from the movement advocating full implementation of the treaty. Instead, he urged Parliament not to make laws from which evil effects flowed. But by 1890, when it became clear that Parliament was not interested in following Tāwhai's advice on the treaty, his political views shifted, and he began to respond to Māori movements outside Parliament. In May 1890, he chaired a meeting of the Northland tribes at Ōmanaia, a place that was spiritually significant to the followers of the mystic Āperahama Taonui, who had been prominent in the treaty movement of the early 1880s. This meeting established an organisational structure that culminated in the formation of the first Māori parliament at Waitangi in April 1892. When there was conflict in the north in the 1890s over the controversial dog tax, Tāwhai continued to urge moderation and adherence to the rule of law, advising his people that the proper course was to petition Parliament to change the law.
Tāwhai had reservations about the writing of Māori history by Pākehā. Nevertheless, he corresponded with the ethnographer Stephenson Percy Smith, supplying him with information on Māori origins in Rangiātea, the Hawaiki origin of the names Waimā-Tuhirangi and Moehau, the ancestral canoe Ōmāmari, the descent lines of Ngāpuhi from Tumutumuwhenua and Nukutawhiti, and Hongi Hika. Tāwhai also intended to write the history of Hongi Hika, but the protracted illnesses which ended his life prevented him from undertaking this work.
Hōne Mohi Tāwhai died on 31 July 1894. He and his wife, Mākere Maraea, had at least two children: Hōne Tākerei Tāwhai and Kereama Tāwhai. Kereama died in 1885 at the age of 21, while studying law with the Auckland firm of Whitaker and Russell.