Mananui was the second of the Te Heuheu line to assume the leadership of Ngāti Tūwharetoa. He was born late in the eighteenth century at Pāmotumotu, near the Mangatutu Stream, the eldest son of Herea Te Heuheu Tūkino I and his first wife, Rangiaho, of Ngāti Maniapoto. Mananui could trace his ancestry to Tamatekapua, commander of Te Arawa canoe, and to its priest, Ngātoroirangi; he was distantly related to both Pōtatau Te Wherowhero of Waikato and Te Rauparaha of Ngāti Toa. He belonged to Ngāti Pēhi (now Ngāti Tūrumakina), Ngāti Hūkere and Ngāti Hinewai hapū, and in his youth lived at Pāmotumotu. During this time his people were at war with Ngāti Maru, Ngāti Hauā, Ngāti Raukawa and Ngāti Whakatere; his first wife, Titi, was of Ngāti Whakatere, but she was sent back to her people because she did not bear a son.
For a time after the death of Mananui's father, Herea, probably in the early 1820s, Ngāti Tūwharetoa lacked an acknowledged war leader. However, Mananui was respected by the elders and chiefs for his outstanding military capacity and his forceful personality. He was closely connected through his mother with Ngāti Maniapoto and Waikato tribes, and with the hapū living on the western and northern shores of Lake Taupō. Further, by marrying two grand-daughters of Te Rangituamātotoru, the predecessor of Herea, he united the eastern and western branches of Ngāti Tūwharetoa. His senior wife was Nohopapa, and the other Te Mare.
With his natural ability to rally and lead, Mananui assumed the leadership of his tribe during the 1820s, when Ngāti Tūwharetoa were under great pressure from the northern tribes newly armed with muskets. He took steps to acquire the new weapons: a large quantity of flax was gathered and taken to Maketū in the Bay of Plenty, where Mananui exchanged it for guns and ammunition with the trader Phillip Tapsell.
The name Mananui, 'great mana', became his when his uncle, the tohunga Taipāhau, transferred his sacred wisdom and magical powers to him on his deathbed, by the ceremony known as ngau taringa. In this ceremony the younger man, who is to be the successor, bites the ear of the older. The spirit of Taipāhau became guardian and guide of Mananui, and all the wisdom of the tohunga passed to him.
The reputation of Mananui as a fighting leader was enhanced through many campaigns against Ngāti Kahungunu, and through the expeditions to the Cook Strait area that he undertook during the 1820s. He travelled with a war party as far as Kapiti in 1825, but declined to join forces with Te Rauparaha on this occasion. Nearly 10 years later, in response to a request from Te Rauparaha for help in settling his disputes with Te Āti Awa, he led a party of 800 men south. In the fighting that followed, Mananui's brother Pāpaka was killed, but Te Āti Awa were defeated at Pākutu. A boundary was set at the Kukutauaki River, near Waikanae. Te Āti Awa were to remain south of the river and Ngāti Raukawa to the north.
Although Mananui asked for a missionary to be sent to him when Bishop G. A. Selwyn visited Ngāti Tūwharetoa in 1843, he had no high opinion either of missionaries or of the British Crown. He scornfully declined missionary invitations to become a Christian, and also their suggestion that he should sign the Treaty of Waitangi. When his brother Iwikau did sign, Mananui repudiated his action. Strong in the remote interior of the country, Mananui pursued tribal interests. In 1841 and 1844 he led war parties to the west coast but was persuaded not to fight. He remained resolutely opposed to British sovereignty and defended the stand by Hōne Heke in the north. In 1845 he told Donald McLean, sub-protector of aborigines in Taranaki, that he believed Heke was 'in the right; that he was asserting his freedom and that of his country…that the British Government intended to deprive the New Zealanders of their lands, their liberty, and their rights as chieftains…. The English were an insatiable people, desirous of conquering all nations…Napoleon Bonaparte would have been a match for them, had he not been taken by stratagem'. The same year he told the missionary A. N. Brown that 'the Queen was not going to take this Island as she had done Port Jackson [Sydney] and other places'. Blocked from the north by other tribes, Mananui could do little for Heke. As a gesture of defiance, however, he strongly opposed any Pākehā settling among the tribe, except traders who did not require land.
When Mananui became leader of Ngāti Tūwharetoa, the tribe's principal pā, Waitahanui, was abandoned, and a new centre established at Te Rapa, on the south-west side of the lake. This was a strong, palisaded pā, where Mananui built a carved house; there he lived with his eight wives. With Nohopapa, his favourite wife, he had three sons and a daughter.
On 7 May 1846 Mananui was killed by a landslide which swept down Kākaramea mountain after heavy rain and overwhelmed Te Rapa. Several members of his family – his second son, Te Waaka, and all his wives – and 54 others died. The only one to escape was Mananui's half-brother, Tōkena Te Kerēhi. His brother Iwikau was living at Waihī, at the southern end of Lake Taupō, at the time, and his son Horonuku (the son of Te Mare), also known as Patatai, was away in Waikato. The body of Nohopapa was found with Mananui's mere, Pahikaure, in her hands.
The bodies of Mananui and Nohopapa were placed in a vault at Pūkawa. Mananui's body was later taken to a burial cave on Tongariro. In 1910 his remains were brought back to Waihī by his grandson, Wī Tamaiwhana, and are buried in the mausoleum on the hill at the southern end of Waihī village. Within a few days Wī Tamaiwhana was killed by another landslide at the very place where Mananui had died. It is said that he had violated a tapu by touching the body of Mananui and then passing by a hangi (on the site of the earlier landslide) which still had scraps of food about it.
Mananui was the most influential leader in the central North Island, and one of the most distinguished Māori of his time. He led his tribe wisely; he forbade the practice of cannibalism; he was respected by his enemies. He was a man of great physical stature, as well as great mana. The artist George French Angas described him in the early 1840s as 'a fine old man with an imposing appearance and dignified carriage; he stands nearly seven feet high and is very corpulent. His hair is silvery white and his people compare it to the snowy head of the sacred Tongariro, there being no object, except this tapu mountain, of equal sanctity to permit of its being mentioned in connection with the head of this chief.'
Mananui is remembered for many sayings and songs. When he was asked why he was so successful in war he replied: 'If you seize a tree by its trunk you cannot move it, but if you seize it by the branches you can shake it, right down to the roots.' The lament he composed on the death of his father, Herea, is one of the sacred songs of Ngāti Tūwharetoa. The first verse of this lament is sung only for direct descendants in the Te Heuheu line.