Moyle was born on 18 July 1929 in Thames. Before entering parliament, Moyle was a secondary school teacher and also had a dairy farm.
Moyle stood unsuccessfully for the Hobson electorate in 1957
He first entered parliament in the 1963 general election, as a Labour MP for the South Auckland electorate of Manukau. In 1969 the Mangere electorate was created in the same general area, and Moyle moved his candidacy there, allowing Roger Douglas to take over Manukau. Moyle was elected for Mangere in the 1969 election, and would hold the electorate for another eight years.
In the 1972 general election, the Labour Party came to power for the first time in over a decade, forming the Third Labour Government led by Norman Kirk. Moyle was appointed to the Ministerial positions of Agriculture and Fisheries, Forests, and Science. In September 1975 he also became Minister responsible for the newly formed Rural Banking and Finance Corporation. He was generally well-regarded, especially as Minister of Agriculture. He was 'enduringly popular with the farming community', and was instrumental in opening up New Zealand's meat trade with the Middle East. As Minister of Forests, Moyle also helped preserve the remaining stands of giant kauri.
In August 1974, Kirk died suddenly, and Bill Rowling took over as Prime Minister and Labour Party leader.
Labour lost power in the 1975 general election, bringing to power the Third National Government led by Robert Muldoon. Many within Labour were dissatisfied with their party's performance under Rowling, and began a campaign to replace him. According to political commentator Bruce Jesson, Moyle was the preferred candidate due to his strong performance as Minister of Agriculture. However any potential leadership coup was derailed due to what became known as 'the Moyle Affair' of 1977.
Muldoon accused Moyle in Parliament of having been questioned by the police on suspicion of homosexual activities, which were then illegal in New Zealand. After changing his story several times, Moyle resigned from Parliament. He later said that he had not been obliged to resign, but had done so because "the whole thing just made me sick". It has been suggested that Muldoon saw him as a leadership threat and acted accordingly. Ironically, the subsequent 1977 by-election was won by David Lange, and the attention that this got him helped propel him to the leadership of the Labour Party and his landslide victory over Muldoon in the 1984 election. In a 1990 interview, Moyle said that the scandal had made him a "sadder and wiser person".
In the 1978 election, Moyle stood for and failed to win the Whangarei electorate. In the 1981 election, Moyle stood for and won the Hunua electorate. This was abolished before the 1984 election, and Moyle stood for, and won, the new electorate of Otara, which he held until his retirement in 1990. In 1984 Labour was again returned to power, forming the Fourth Labour Government under David Lange. As one of the few Labour MPs with Ministerial experience, Moyle was reappointed to Cabinet, again holding the portfolios of Agriculture and Fisheries (now separate departments) and regaining charge of the Rural Banking and Finance Corporation.
The government's policy was market liberal and reformist. Driven by Finance Minister Roger Douglas, it embarked on a programme, known as Rogernomics, aimed at deregulating the economy. Moyle's portfolio of Agriculture was strongly affected by this, as the farming sector had been one of New Zealand's most heavily subsidised. In the 1982-83 financial year, for example, it has been estimated that farm subsidies cost 'well over' a billion New Zealand dollars. Under the Fourth Labour Government, virtually all state financial assistance was removed from agriculture. Moyle was a supporter of the reforms, but was not associated with them to the same extent as many of his colleagues despite their effect on his portfolio.
The fishing industry was also overhauled at this time. In particular, a Quota Management System was introduced in order to manage the country's fishing stocks. Because this initially made little provision for traditional or other Māori fishing rights, it was challenged by the Waitangi Tribunal and several iwi. Under Moyle, a Maori Fisheries Act was introduced to deal with this, recognising Māori rights to a share of fisheries and the fishing industry.
Although involved in several important reforms, Moyle had a low profile in the government, avoiding publicity. At the 1987 election he had announced that he would probably retire from parliament at the 1990 election, and in 1989 he confirmed this. Along with other Ministers who had announced their retirement, Moyle was dropped from Cabinet by Prime Minister Geoffrey Palmer in early 1990. He had wanted to keep his Ministerial position until that year's election in order to complete the restructuring of the meat industry.
In the June 1990 Queen's Birthday Honours List, Moyle was awarded a CBE for public service.
Moyle is a convert to Roman Catholicism.