Robert Tahupotiki Haddon was born on 5 October 1866 in the Hokianga district, the son of Charles Haddon, a Scots bushman and farm labourer, and his wife, Te Paia (Sophia) Tahupotiki, of Nga Ruahine and Ngati Manuhiakai, hapu of Ngati Ruanui; her ancestors had also intermarried with Nga Rauru. Her great-great-grandfather was Titokowaru, grandfather of Riwha Titokowaru, the visionary war leader of the late 1860s. It is likely that Haddon was baptised a Wesleyan with the name Robert; he was known to both Maori and Pakeha as Tahupotiki.
As a girl, Haddon's mother was trained as a domestic servant and instructed in religion in the homes of some of the earliest Wesleyan missionaries in Hokianga. She passed on her teaching and faith to her children. Te Paia died when Haddon was in his teens and he took over the responsibility for the younger children of the family. Possibly at her death they returned to her tribal lands in Taranaki; in his late teens Haddon became associated with Parihaka. He was informally adopted by his kinsman, Tohu Kakahi, and trained in his beliefs and traditional knowledge. On 8 September 1896, at Waverley, Haddon married Huihana (Susan) Haerehau Shelford, daughter of Edward Oliver Shelford and Iritana (Eritana) Raiehe, also known as Te Paenga. They were to have five daughters and four sons.
The superintendent of the Wesleyan Methodist church's West Coast Maori mission, T. G. Hammond, was in the habit of visiting Tohu at Parihaka, and there he encountered Haddon. He took him into his home for a period and continued his education. Haddon was accepted by the church's annual conference of 1898 as a candidate for ministry, and began two years of theological training at Wesley College, Three Kings, Auckland. In addition to theology, the programme of study also laid emphasis on accepting the ideas of the Te Aute College Students' Association, later part of the Young Maori Party.
Haddon was appointed a probationer in 1900, working under the supervision of Hammond in the Taranaki district and based at Patea. His early years were difficult, especially at Parihaka, where he was regarded as a traitor by Tohu's followers and by the still-embittered victims of land confiscation and government attacks on Parihaka; he was frequently mocked in the streets. There were fewer than 30 Maori Methodists in his district and the number was declining. His problems lessened and his influence increased after the deaths of Te Whiti and Tohu in 1907.
About 1902 Tahupotiki Haddon was moved from Patea to Okaiawa. He was ordained a minister in 1904, and almost immediately was sent to spend four months visiting Methodist congregations in the South Island in an unsuccessful attempt to amalgamate its Maori and Pakeha circuits. When the Presbyterian church opened its Turakina Maori Girls' School in 1905 the Methodist church had no similar school, so Haddon made trips through Taranaki and Waimarino seeking pupils for Turakina among Methodist families. He was to continue to promote the idea of a Methodist school for girls in Taranaki. In 1917 he was appointed to a sub-committee of the Methodist conference to explore the question, but his aim was achieved only after his death, when Rangiatea, a Methodist school of domestic science, was opened in New Plymouth in 1940.
Haddon developed into a formidable orator; his rank and his impressive physical attributes – he was tall and distinguished in appearance – ensured his gradual acceptance, helped by the fact that he shared the land concerns of his people. In 1909 he joined a west coast union led by Waata Wiremu Hipango; it was intended to protect Maori interests and work for the return of the confiscated lands. The same year he was one of a delegation of 70 Maori to Wellington protesting against the granting of perpetual leases on the West Coast Settlement Reserves, and demanding the return of the lands when the present leases expired. When Taranaki Maori were seeking a candidate for the Western Maori seat in 1911 Haddon was asked if he would stand, but replied, 'I cannot serve two masters.' Taranaki chose Maui Pomare.
In 1910 Haddon had been moved to Normanby. He was often absent and had little home life. By 1918 he was circuit superintendent in Taranaki–Waimarino. A major part of his work was done on temperance crusades on the west coast and further afield, and up to the 1920s, at the request of the New Zealand Alliance, he was frequently released from his usual duties for this work. For years he visited all the major hui up and down the country with his message, canvassing support for prohibition. In July 1923 he was a leading member of a Maori deputation to Parliament petitioning against the introduction of liquor licences in the King Country.
When Haddon's kinsman, Tahupotiki Wiremu Ratana, began his faith healing and religious revival mission, Haddon was an enthusiastic supporter. In 1920, when Ratana opened his church, Piki-te-Ora, at Ratana pa, the interdenominational religious services were arranged by Haddon. At the Methodist annual conference in Wellington in 1921 Haddon described Ratana's mission as 'out and out Christianity'. There was no overt clash between his Methodist ministry and his role as a Ratana follower because A. J. Seamer, general superintendant of the Methodist Maori mission, thought it best to avoid conflict with the new organisation.
Even after the Ratana church was registered in 1925, Haddon, by this time senior Methodist Maori superintendent, was free to continue his association with Ratana. In 1927 his eldest son, Oliver (Oriwa), who had followed him into the Methodist ministry, became a teacher at the primary school at Ratana pa. In the same year Haddon, perhaps because his cousin was facing a series of crises, publicly declared his support for Ratana by offering to become the first person baptised in the new temple then being built at Ratana pa.
After the death of Maui Pomare in 1930, Haddon offered himself as the candidate for Western Maori. As the battle developed between the Ratana candidate and Te Taite Te Tomo, Haddon withdrew. At this time he was based in Ngaruawahia where, under the leadership of Te Puea Herangi, there was a Maori community of growing importance. The Methodist church acquired a parsonage there and in 1930 Haddon, regarded as the chief Maori Methodist pastor, was appointed to occupy it. As it had done with Ratana, so the Methodist ministry planned to do with the King movement: work away quietly without overt opposition to Maori forms of religion. Haddon made no objection to attending Pai Marire services. Te Puea regarded Haddon as a 'splendid man', and he became an ally of hers in her many schemes of reform. After initially ignoring him, the Ngaruawahia people increasingly called on his services for weddings and funerals. Haddon took a leading role at the tangihanga of Te Rata and in the installation of Koroki as the fifth King in 1933. On this occasion, to pay Haddon honour, the elders of the royal family dressed him in a feather cloak that had belonged to Tawhiao, the second King.
In 1932 Haddon returned briefly to Normanby; his wife Huihana died there in August. He was then stationed in Te Kuiti for two years. In 1934 he was a guest at the important Waitangi hui; Taranaki Maori presented a picture of the signing of the Treaty of Waitangi, painted by his son Oliver, to be hung in the Treaty House. In 1935 he returned to Normanby but, though approaching 70, was constantly travelling, conducting missions and attending important hui. In 1935 he conducted two special missions in Northland with Eruera Te Tuhi. In 1936, with Seamer, he escorted the Methodist Maori Choir to Australia.
Late in 1936 Haddon was visiting one of his married daughters at Matakana. He fell ill and was taken to Warkworth Cottage Hospital where he died on 5 November. As the body was being brought home to Taranaki for burial, at the request of King Koroki a halt was made at Waahi and the body taken into the meeting house. Koroki covered the coffin with two precious cloaks and farewell orations were given. Haddon was a Freemason, and the funeral was conducted partly according to their rites. Tahupotiki Haddon was buried at the urupa (burial ground) at Weriweri, near Normanby, on 8 November. In 1948 a new Methodist Maori youth centre at Hawera was dedicated to his memory.