Mohi Te Ātahīkoia was born probably at Waimārama, Hawke's Bay, in the early 1840s. His mother was Maata Kōtakitaki, whose parents were Ngāti Whakaiti chief Tūāhu and his wife, Meretuhirangi. His ancestor Whakaiti was descended from Kahungunu, from Te Aomatarahi of Ngāi Tahu and Ngāti Ira, and from earlier tribal groups occupying Hawke's Bay. The territory of Mohi's two main hapū, Ngāti Whakaiti and Ngāti Kautere, lay in the Waimārama district. At the time of Mohi's birth, the greatest chief there was Tiakitai of Ngāti Kurukuru. Living under his protection was a European whaling community at Rangaiika, south of Cape Kidnappers. Mohi's father is thought to have been Portuguese; family genealogies give his name as Kupa.
As a very young child Mohi was taken into the care of Tiakitai. After Tiakitai's death in 1847, he was fostered at Waimārama by his aunt, Kāterina Morurangi of Ngāti Hikawera, from Wairarapa. Her husband was Pāora Whatuira, Maata's brother. In 1849 many Waimārama people were baptised by the missionary William Colenso, and it is probable that Mohi was among them. In his boyhood he received training from his grandfather, Tūāhu, in the lore and traditions of his people. Tūāhu had succeeded Tiakitai as principal chief at Waimārama and Mohi expected to be his heir. However, Te Teira Tiakitai, son of Tiakitai, came home from Poverty Bay in April 1850, and because of Mohi's youth Tūāhu encouraged Te Teira to take his father's place. This led to a lifelong rivalry between Mohi and Te Teira.
In 1858 Mohi's foster parents took him to Ngā-waka-a-Kupe, north-east of Lake Wairarapa. They built a house at Uweroa, and lived there for three years before returning to Waimārama. Around 1860 Mohi married Ema Te Owha, daughter of Eru Kaimokemoke. Their eldest son was Pita Te Atua Te Ātahīkoia Mohi; they were to have five other children. Later Mohi took a second wife, Pukepuke Tangiora of Ngāti Te Mihiroa. The child of this marriage was Te Ākonga. Mohi lived sometimes with his first family at his house on a knoll overlooking Waipuka (Ocean Beach), north of Waimārama, and sometimes at Pakipaki with his second wife. Ema died in 1924, and Tangiora in 1936.
In 1868 and 1869 Mohi joined the Ngāti Kahungunu force which supported government troops in the pursuit of Te Kooti in the Urewera and the Taupō area. He was present at the last pitched battle with Te Kooti, at Te Pōrere, on 4 October 1869, where he and a few others rushed into the pā; Mohi captured two women.
When the Waimārama, Ōkaihau and Waipuka blocks first appeared before the Native Land Court in 1868, Mohi was a grantee and his mother was listed among the owners of Waimārama. But in 1884 Te Teira Tiakitai wished to subdivide the blocks and claimed that the mana belonged to him alone. Mohi established that his ancestors had lived at Waimārama with those of Te Teira, and insisted that Tūāhu had been principal chief and was the equal of Tiakitai; in a partial victory his people were awarded land in the Waimārama and Waipuka blocks. As a result of his experiences in the land court, Mohi became increasingly involved in the political and land struggles of his time. He accompanied Hēnare Matua to Whanganui in 1874, where Matua expressed the views that land selling and leasing should cease and Māori representation in Parliament should be increased. Mohi then visited Ngāti Hikawera chiefs Wī Hikawera and Hāmuera Tamahau Mahupuku in Wairarapa, remaining there until 1875, when he returned to Waimārama.
By 1891 Mohi was chairman of the Hawke's Bay Native Committee. He addressed the Native Land Laws Commission at Waipāwa that year, outlining his people's problems with Pākehā leases of their land. In 1906 he appealed to the premier, Richard Seddon, to have the Waimārama land preserved for the support of local Māori. In 1907 the leases were investigated by Robert Stout and Apirana Ngata's Native Land Commission, and Mohi stated that he did not wish to sell his interests.
Mohi became involved with Te Kotahitanga, the movement for an independent Māori parliament, from its preliminary session in the Bay of Islands in April 1892. He became one of the movement's leaders, acting as its emissary and spokesman, and taking a prominent part in debates. He took an active role from the second session of the Kotahitanga parliament at Waipatu in 1893. He was Speaker of the Great Council of the parliament for a period, and was nominated by its premier, Hāmiora Mangakāhia, to be chairman. Te Keepa Te Rangi-pūawhe was elected, however. Later, despite efforts to get him to change his mind, Mohi withdrew as Speaker, because he wanted to feel free to give his own opinions.
In 1897 Mohi was one of the Kotahitanga leaders who accompanied Tamahau Mahupuku to Wellington to interview Seddon about a petition to Queen Victoria requesting the inalienable reservation of remaining Māori land to Māori. Seddon proposed the setting up of native land boards, but Mohi opposed Seddon's consequent Native Lands Settlement and Administration Bill of 1898. He appeared before Parliament's Native Affairs Committee that year, where he invoked the Treaty of Waitangi, claimed that 20,000 Māori were opposed to Seddon's bill, and demanded that they have the right to make their own decisions on leasing their remaining lands.
The eighth session of the Māori parliament met at Waitangi, Bay of Islands, in 1899. Mohi was one of four leaders chosen to travel about the country to solicit further support. He also unsuccessfully contested the Eastern Māori seat in the House of Representatives against Wī Pere that year. In 1900 Mohi adopted a more moderate line, accepting that the Māori Lands Administration Act and Māori Councils Act passed as a consequence of Seddon's 1898 bill were an acceptable compromise to Māori demands for control over their lands.
After Hēnare Tomoana's death in 1904, Mohi was recognised as a senior chief and leader of Ngāti Kahungunu. He took over the leadership of the Ngāti Kahungunu campaign for the title to Te Whanganui-o-Orotū (the Napier inner harbour) and the Puketitiri reserve. In 1918 he gave evidence to the Native Affairs Committee, and in 1919 he petitioned the government. As a result, a commission of inquiry was held in 1920. Although the claim was not successful at the time, it was revived in 1945, 1948 and again in 1988.
Mohi had collected all the genealogies of his own people of Waimārama and, as his interest developed, of greater Hawke's Bay. His reputation as a historian grew, and in July 1907 the Tānenui-a-Rangi committee of Wairarapa elders, set up to collect the traditions and whakapapa of their people, requested a copy of Mohi's book. Mohi also wrote a long history of Waimārama and Hawke's Bay, entitled 'Ko tēnei kōrero nō Hawaiki rānoa'. It begins in Hawaiki, traces the journey of the Tākitimu canoe, the travels of Tamatea and Kahungunu, and the migrations of Kahungunu's children from Poverty Bay to Hawke's Bay. It recounts the struggles between the migrants and the local inhabitants, the peace treaties which permitted them to dwell together, and the battles between Ngāti Te Whatuiāpiti and Ngāti Kahungunu, which led to migrations to Wairarapa and beyond. The history continues up to Mohi's own involvement in the fighting against Te Kooti.
Mohi Te Ātahīkoia died at Pakipaki on 30 June 1928. He was buried at Waimārama. The inscription on his grave reads, simply, 'He kaiārahi nō te iwi' (A leader of his tribe).