Henry Greathead Rex Mason was born on 3 June 1885 in Wellington, the son of Harry Brooks Mason, a compositor from Cape Town, and his Australian-born wife, Henrietta Emma Rex, who helped to form the Women's Social and Political League in 1894. Rex Mason was dux of Wellington College in 1902 and graduated MA with honours in mathematics from Victoria College in 1907. He then graduated LLB. In 1911 he moved to Pukekohe, where he set up in practice as a lawyer. On 27 December 1912, at Auckland, he married Dulcia Martina Rockell; they were to have two sons and two daughters. Dulcia interested her husband in Indian religions, and both were theosophists, vegetarians and teetotallers.
In 1915 Mason was elected mayor of Pukekohe and in the 1919 general election, having joined the New Zealand Labour Party after it was founded in 1916, he stood as its candidate for the Manukau seat. By the 1920s he was an influential member of the Auckland Labour Representation Committee and in 1922 and 1925 stood as candidate for Eden, very narrowly losing in 1922. He finally won the seat at a by-election in April 1926, the vote for the Reform Party having been split by the independent, ex-Reform candidate, Ellen Melville. He was to represent the seat and its successors (Auckland Suburbs, Waitakere and New Lynn) for 40 years. For some time Mason remained involved in local body politics. He was a member of the Auckland Transport Board between 1931 and 1939 and its chair from 1935 to 1939. He narrowly failed to win the mayoralty of Auckland in 1933.
Mason was keenly interested in the workings and organisation of the Labour Party and in 1931 was elected its president. After 1931 he became involved in the party's policy committee, which prepared the policy for the 1935 election, consolidating its shift away from socialism towards laying the groundwork for a welfare state in New Zealand. This reflected Mason's own social democratic rather than socialist politics. He also advocated that citizens over 55 should be paid decent pensions, welfare benefits should be paid to the unemployed and incapacitated, producers should receive a fair income, all employees should have full pay during a fortnight's annual holiday, and necessary public works should be advanced.
In common with some other Labour MPs, and many of the party's supporters, Mason believed firmly in the sole right of the state to create and issue credit and currency. In the early 1930s he was present at meetings which seem to have consolidated Labour support among farmers influenced by Douglas Social Credit. In 1934 he wrote a short pamphlet, Common sense of the money question. The booklet echoed many of his early speeches: he had called the Reform government of the 1920s 'essentially a money-lenders' Government'. Mason forged a close alliance at this time with another MP, Frank Langstone. In 1933 they jointly put to the Labour Party conference proposals for the Reserve Bank of New Zealand to create credit. The party's chief financial spokesman, Walter Nash, took a more cautious approach and in the end won out. Mason never forgave him, and the issue of monetary reform proved a barrier to their becoming good friends.
In 1935, after Labour's sweeping victory, Mason was given the portfolios of attorney general and minister of justice. In 1937 he established the Law Revision Committee, responsible for later important reforms. After the 1938 election, amid increasingly acrimonious debates over caucus democracy and financial policy, he remained close to the prime minister, Michael Joseph Savage. But he also retained contacts with John A. Lee, Langstone and the other monetary reformers. When Lee was expelled from the party in 1940 after his attacks on Savage, Mason wrote to him and asked whether or not he should join Lee's breakaway organisation. He accepted Lee's advice to work from within the party.
From 1940 to 1947, under Peter Fraser's prime ministership, Mason was minister of education and from 1943 to 1946 native minister. In 1944 he published a booklet, Education today and tomorrow, which described the new state school system and was obviously strongly guided by his director of education, C. E. Beeby. The matriculation examination was abolished, and a resulting reform of the secondary school curriculum resulted in the breaking down of differences between secondary and technical high schools.
After the 1946 election, narrowly won by Labour, Mason was not elected by his colleagues in the ballot for cabinet. However, he was elected to fill a vacancy in April 1947. After the defeat of Labour in 1949, Mason continued to live in Wellington, travelling to Auckland for a monthly constituency clinic. He was an energetic opposition MP, speaking on many subjects including law reform and monetary policy. Several times between November 1950 and April 1956 he introduced the Decimal Coinage Bill into Parliament as a private member's bill. His persistence, usually without the support of his colleagues, prepared the way for the eventual introduction of decimal currency into New Zealand in 1967.
In 1957 Mason waged a particularly energetic campaign in Waitakere and was returned with a large majority. Some of his strongest supporters were members of the Yugoslav community. The first Labour government had made all naturalised New Zealanders eligible for such benefits as the old-age pension. The second Labour government promised more sensible liquor laws so that wine could be sold from an increasing number of outlets, mostly in West Auckland; this would benefit Yugoslav wine-growers. Rex Mason received much of the credit for these measures as minister of justice.
After Labour won the election in November 1957 Mason was easily elected to cabinet. He again became attorney general and minister of justice, and also served as minister of health. His major achievement was to codify all aspects of criminal law and to prepare a crimes amendment bill that was eventually passed in 1961 by the incoming National government. Considered one of the best codifications of its type, it also abolished the death penalty for murder.
By 1963 the electorate of Waitakere had grown in size so much that a new seat, New Lynn, was created, which Mason won with a comfortable majority in that year's election. Between 1963 and 1966 the Labour Party – first under Arnold Nordmeyer, and more importantly after 1965 under its new, energetic and very much younger leader, Norman Kirk – wanted to modernise itself. Many of the stalwarts of the first Labour government, including Mason, were now in their 80s and still in Parliament. In 1966 a policy was adopted whereby members of Parliament should retire at the election after they turned 70. Despite protests from some elements of the party organisation, a new candidate was selected for New Lynn in early 1966.
Mason retired from Parliament in October 1966. He was appointed a CMG in 1967 and received an honorary LLD from Victoria University of Wellington in the same year in recognition of his service as attorney general and his long interest in law reform and codification. Dulcia Mason died in 1971 and Rex on 2 April 1975 at Wellington. He was survived by two daughters and a son. He had outlived nearly all his colleagues from the first Labour government. Mason had contributed much to the direction of both the first and second Labour governments and to the creation of a welfare state in New Zealand.