Henry Greathead Rex Mason CMG QC (3 June 1885 – 2 April 1975) was a New Zealand politician. He served as Attorney General, Minister of Justice, Minister of Education, and Minister of Native Affairs, and had a significant influence on the direction of the Labour Party. The longest-serving Member of Parliament in New Zealand history, Mason served in Parliament continuously from 1926 to 1966. He is also the only person to serve as an Member of the New Zealand Parliament for over 40 years.^[1]

Mason was born in Wellington on 3 June 1885.^[2] His father was Harry Brooks Mason, a compositor at the Government Printing Works (who worked for Hansard for a time) from South Africa. His mother, Henrietta Emma Rex, was an Australian who helped form the Women's Social and Political League and was vice-president in 1894. She also taught ballroom dancing in Wellington prior to World War I.^[3]

Mason was educated at Clyde Quay School, then Wellington College where he was dux in 1902. He won a scholarship and attended Victoria University where graduated in 1907 with a Master of Arts with honours in mathematics and a Bachelor of Laws. He then worked in several law firms in Wellington and Eltham before opening his own practice in Pukekohe in 1911. He was soon joined in the practice by his brother Spencer, who later became president of the Auckland District Law Society.^[3]

On 27 December 1912 Mason married Dulcia Martina Rockell at Auckland. Together they had two sons and two daughters.^[3] Through his wife's influence, Mason become interested in Indian religion and spirituality, and beliefs derived from it (particularly Theosophy). He was a vegetarian and a teetotaller.^[2]

Mason was elected Mayor of Pukekohe in 1915. He was left-wing in his political outlook, and joined the Labour Party on its foundation in 1916. In the 1919 general election, he was Labour's candidate for the seat of Manukau, but was defeated. Mason sought the Labour nomination for the 1921 by-election in the Auckland East seat, but was beaten by John A. Lee.^[4] Later, he shifted his attention to the seat of Eden — he contested it in the 1922 election and 1925 election. He finally won Eden in a 1926 by-election, assisted by the fact that the Reform Party's vote was split by a defeated nominee, Ellen Melville.

Rex Mason represented the seat of Eden in the 22nd Parliament (1926–28), Auckland Suburbs in the 23rd to 27th Parliaments (1928–46), Waitakere in the 28th to 33rd Parliaments (1946–63), and New Lynn in the 34th Parliament (1963–66).

In both 1931 and 1933 he stood unsuccessfully for Mayor of Auckland City on a Labour Party ticket, defeated by George Hutchison on both occasions.^[5][6]

Throughout his parliamentary career, Mason remained highly involved in the organisation of the Labour Party. He served as its president from 1931 to 1933, and played a major role in policy formulation. Mason was regarded as a social democrat rather than a socialist, and he played a part in moving the Labour Party closer to the political centre. He did, however, believe that the state should have exclusive control over the country's financial system, influenced by social credit monetary reform theories. Other causes supported by Mason include the establishment of a comprehensive old-age pension system and the granting of full state services to naturalised immigrants (the latter making him extremely popular with his electorate's substantial Yugoslavian community).^[2]

In 1935, Mason was awarded the King George V Silver Jubilee Medal.^[7]

Mason died in Wellington on 2 April 1975, aged 89, and his ashes were buried in Karori Cemetery.^[15][16]

Mason Street in his home electorate of New Lynn is named after him.