Tonga Mahuta was born probably in 1897 at his father's home at Hukanui, near Waahi pā, Huntly. He was the fourth surviving son of Mahuta Tawhiao Pōtatau Te Wherowhero, the third Māori King. His mother was Te Marae, a daughter of the chief Amukete (Amuketi) Te Kerei, who had been killed during the fighting at Rangiriri in November 1863. Tonga's eldest brother was the fourth King, Te Rata. His main tribal affiliation was with Ngāti Mahuta, but the family was connected by senior lines of descent to many major tribal groups. Little is remembered of Tonga's upbringing. He was educated at Rākaumanga Native School until 1908, and thereafter at schools in Huntly, Thames, Cambridge and Auckland.
In 1914, when the First World War commenced, Tonga was in his late teens. The leaders of the King movement were reluctant to allow their young men to volunteer for service in the New Zealand Expeditionary Force because the issue of the confiscation of Waikato lands had not been resolved, and because they held to the revived Tariao form of the Pai Mārire religion. Tonga Mahuta refused to report for territorial training in 1914 and was fined. In early 1916 he was prosecuted again with a few others from Waahi, selected from among leading families in the hope that the example would break resistance. He was fined five shillings. In April 1916 he was prosecuted again. On 26 June 1916 Major E. H. Northcroft visited Waahi. The King movement leaders there reproved him; they were concerned that Tonga, a member of the Māori royal family, was being pressured about territorial training when they saw Europeans of similar status exempt. Tonga was prosecuted again in April 1917.
In June 1917 a proclamation extending the provisions of the Military Service Act 1916 rendered Māori liable to be called up for military service to the Native Expeditionary Force Reserve. The minister of defence, James Allen, reassured Te Arawa petitioners and other 'loyal' tribes that the measure would be used only against Waikato.
Tonga Mahuta was called up in the ballot of 25 June 1918; his name had been confirmed through use of confidential census information. On 7 June three constables had gone to Te Paina, Mercer, where the King movement people were debating conscription, and arrested seven Māori, including Tonga's younger brother Te Rauangaanga. He was taken to Narrow Neck camp, Auckland, where Māui Pōmare, his member of Parliament, and Colonel G. W. S. Patterson were able to persuade him to take the oath of allegiance and to write a letter, later distributed among Waikato Māori, saying that it was better to go into camp and not to flout the law. Probably because of his brother's letter, when Tonga was arrested on 24 August he called out to the 44 taken with him to accompany him to camp, without resistance.
Thirty-four Waikato Māori arrested with Tonga consistently refused to wear military uniform or to train, and were punished with 'dietary deprivation' (bread and water) and various periods of imprisonment. Six were court-martialled and sentenced to two years in gaol with hard labour. It was believed by the military authorities that Tonga's cousin Te Puea Hērangi had Prussian ancestry through her grandfather, William Searancke, and that Waikato Māori were German sympathisers. Perhaps to dispel this notion, and out of support for Te Rauangaanga, by September Tonga had consented to wear uniform and to train. Not long afterwards the war came to an end, and by June 1919 all trainees at Narrow Neck and all prisoners were released. Outstanding warrants for arrests of Māori were not executed.
Tonga Mahuta married Te Okewhare, the daughter of Te Awaiti Hiko and Te Aira Hiko, both of Ngāti Naho. There were at least three sons and three daughters of the marriage. Tonga was interested in rugby league as a young man, and was later made a life member of the New Zealand Rugby Football League. His interest in sport continued, and by 1925 he was a member of the Māori Advisory Board of the New Zealand Rugby Football Union.
King Te Rata was often either ill or unwilling to meet official visitors, and his brothers, including Tonga, deputised for him. During the struggles that raged among leaders of the King movement over the Rātana movement, Tonga and his brothers gradually came to support Te Rata and Te Puea rather than the Rātana faction led by Piupiu Te Wherowhero and Tupu Taingakawa. They also came to accept the plans of Apirana Ngata concerning land consolidation. By 1929 Ngata considered that Tonga Mahuta was a progressive leader who would teach Waikato what was possible in the way of land management. In February and March 1932 Tonga accompanied Te Rata to Kawhia to encourage land consolidation schemes, and organised the development of 600 acres for Te Rata and his other brothers. After Te Rata's death in 1933, Tonga and his brothers competed with Te Puea for influence over the young fifth King, Korokī.
During the Second World War, Tonga Mahuta was more supportive of the war effort than some Waikato people would have liked. Te Puea and other King movement leaders permitted individual young Waikato men to enlist if they chose, but the confiscation issue was still not settled: the compensation offered in 1928 had been refused, and Waikato felt that their young men had little reason to be loyal to the government. They were willing to allow their men to defend New Zealand itself, and to assist with the growing of extra food, but little more. Tonga Mahuta supported this line, but took it a little further, actively encouraging Waikato Māori to join the Home Guard. After a meeting in Wellington in which Te Puea and other leaders discussed with Prime Minister Peter Fraser the Waikato attitude to the war effort, Tonga organised the voluntary transfer of Waikato people into industries such as the coalmines and freezing works. He encouraged his people to raise essential foodstuffs and to contribute to patriotic funds. For a period he was associated with the National Service Department in Hamilton.
The confiscation issue was temporarily laid to rest in 1946 with the acceptance by Te Puea and other King movement leaders of a monetary settlement. The Tainui Māori Trust Board was set up to administer the annual payments. When the first funds arrived Tonga suggested that each year the money should be used to purchase a farm for each of the many hapū affected by the confiscation. His suggestion was not carried out. Tonga Mahuta died of tuberculosis at Waahi pā, on 13 March 1947. His body was taken from Waahi to Tūrangawaewae for a huge tangihanga, and he was buried on Taupiri Mountain. Te Okewhare had died on 13 March 1944, and Tonga was survived by six children.
All his adult life Tonga Mahuta had lived at Waahi in the home formerly occupied by his father, King Mahuta. He had been one of a group of Waahi leaders who, while recognising Te Puea's role as one leader of the King movement, resisted her efforts to monopolise authority. He had assisted the King movement to maintain its separate identity and to continue its development as a major force in the Māori world.