Political career
MacIntyre represented the Hastings electorate in Parliament from 1960 to 1972. With Robert Muldoon and Peter Gordon he was one of the three "Young Turks" of the National Party who entered Parliament in 1960. The trio would regularly discuss the issues of the day over late-night drinks and their wives also were to become friends. They became noted for their willingness to challenge senior National ministers as well as members of the Labour opposition on issues. In 1961 he was one of ten National MPs to vote with the Opposition and remove capital punishment for murder from the Crimes Bill that the Second National Government had introduced.
He served under three Prime Ministers (Keith Holyoake, Jack Marshall, and later Robert Muldoon) as Cabinet Minister. He was Minister of Lands and Minister of Forests from 1966 to 1972, Minister of Māori Affairs, Minister of Island Territories from 1969 to 1972 and Minister of Island Affairs and Minister for the Environment in 1972. He gained a reputation as capable and hardworking and was a strong advocate for diversifying forest planting as well as its applications on private land, seeing its potential to boost the incomes of farmers as well as providing employment and supporting the processing industry. In 1971 and 1972 MacIntyre was Administrator of Tokelau.
He promoted the Māori Affairs Amendment Act 1967, which threatened to increase the alienation of Māori land, but MacIntyre foresaw that there would be economic opportunities for Māori in both the forestry and fisheries industries. He would emphasise 'self-help' to Māori and urged iwi and hapū to develop their lands productively to avoid the risk of losing them.
As Minister for the Environment he became unpopular during the Save Manapouri campaign and, just weeks out from an election, gazetted trout farming legislation which displeased anglers and conservationists. At a public meeting in Hastings during the election campaign the prime minister, Jack Marshall, told those in attendance "If anglers want the trout farm legislation repealed, then I suggest they vote for Mr. Kirk [Labour Party leader Norman Kirk]." They did and not only was National beaten heavily but MacIntyre was defeated in Hastings at the 1972 election. Out of parliament he returned to farming, before being appointed by the Meat Producers' Board as chairman of their export committee.
After three years He returned to parliament to represent the Bay of Plenty electorate in Parliament from 1975 to 1978, and the East Cape electorate from 1978 to 1984.
Under Muldoon he was appointed again to the cabinet. He was Minister of Māori Affairs (1975–1978), Minister of Agriculture and Fisheries (1975–1977), Minister of Agriculture (1977–1984), and Minister of Fisheries (1978–1984). He was made a Privy Councillor in 1980.
In September 1980 MacIntyre gave a Marginal Land Boards loan to his daughter and son-in-law raising questions around Conflict of interest. In 1979 his daughter Audrey and her husband, Jim Fitzgerald, had purchased land near Karori, Wellington. After applying twice unsuccessfully, they were granted a loan of $137,000 from the Marginal Lands Board to develop the property. A public inquiry later concluded that MacIntyre had not acted willfully improperly. Nonetheless, several public resignations of National Party officeholders followed in MacIntyre's East Cape electorate causing serious damage to MacIntyre's reputation. He later conceded that the 'Marginal Lands affair' was "a ghost that haunts me for the rest of my life."
By 1982, MacIntyre had a serious heart problem and he decided to retire at the 1984.
