Frank Langstone was born, probably on 10 December 1881, in Bulls, the fourth of five children. His father, Charles Walter Langston, had various occupations, including that of veterinary surgeon; his mother, Margaret McDermott, was a seamstress. She died on 23 December 1890, his father having deserted the family sometime earlier. It was left to Frank's sister Katherine to look after the children. The schooling he received was poor and he later went into foster care before being apprenticed to a blacksmith. Nevertheless, he read extensively.
Around 1906 Langstone became the proprietor of the railway refreshment rooms in Masterton. He married Agnes Clementine King at Masterton on 24 April 1906; they were to have five sons and two daughters. He also became involved with the Wellington branch of the New Zealand Shearers' Union and the first New Zealand Labour Party. Langstone later ran a billiard saloon in Masterton before moving to Te Kūiti around 1913 to run another restaurant. He was president of the local branch of the Social Democratic Party and in 1916 joined the newly formed New Zealand Labour Party. He lived in Auckland for a time before returning to Te Kūiti in 1918, again to run a restaurant; he managed another in Taumarunui from 1919.
At the 1919 election Langstone stood unsuccessfully as Labour candidate for the Waimarino electorate. He continued as a restaurateur and helped the New Zealand Workers' Union to organise the local timber workers, before winning the seat in 1922. His maiden speech to Parliament in 1923 emphasised themes he was to return to consistently throughout his parliamentary career: developing agricultural land, financial security for small farmers, and the creation of a state bank. A brilliant orator, with a tenacious memory, he soon fitted into the demands of parliamentary life. In 1925 Langstone narrowly lost Waimarino, and returned to the restaurant, which Agnes had managed while he was in Parliament. After unsuccessfully trying to gain the Labour nomination for Eden in 1926, he won back Waimarino in 1928.
Langstone was elected president of the Labour Party in 1933, but living in the King Country prevented him from being fully involved in the party's policy formation. Nevertheless, in 1934 and 1935 he published several pamphlets setting out Labour's plan for a guaranteed price for agricultural produce, supporting state control of the issue of currency and credit, and calling for the maintenance of a stable internal price level by matching money supply to production and consumption. Not satisfied with the creation of the Reserve Bank of New Zealand in 1933, he wanted to nationalise all deposit banks. Although influenced by the monetary theories of Major C. H. Douglas, he opposed some of his ideas and argued that Douglas did not go far enough in criticising capitalism.
Following Labour's election victory in 1935, Langstone was appointed minister of lands and commissioner of state forests. He was particularly concerned with afforestation, soil erosion and river control. His administrative abilities impressed civil servants with whom he worked. In June 1936 he led a goodwill mission to Western Samoa and in 1939 represented New Zealand at the International Labour Organisation in Geneva. From 1940 he was minister of external affairs and minister for the Cook Islands. A competent Māori speaker and generally supportive of Māori concerns, he also assisted Prime Minister Michael Joseph Savage with the native affairs portfolio until 1940 and became minister on Savage's death. Despite a policy favouring equality between the races, and the ending of some discriminatory practices, few new initiatives were taken. While acting minister in 1937 during Savage's absence overseas, he and John A. Lee tried to oust Māori from their Ōrakei village site on the Auckland waterfront. Savage, who had promised Māori they could keep the site, reversed the decision on his return.
Within cabinet Langstone continued to support state control of credit to provide low-interest loans for farms, houses and industry. He was sympathetic to the group, led by Lee, that wished to change government policy and the personnel of cabinet. He retained his friendship with Lee after the latter's expulsion from the party in March 1940, and later contributed to his newspaper, John A. Lee's Weekly. Langstone became increasingly restive over Labour's financial policies, the retention of the Legislative Council and plans for a bipartisan war cabinet.
In June 1941 Langstone was asked by Prime Minister Peter Fraser to go on a mission to Washington DC to sell primary produce and to set up a New Zealand Legation. However, any thoughts he may have had of heading the legation were dispelled when Walter Nash was appointed in November 1941. In April 1942 Langstone became high commissioner for New Zealand in Canada. Six months later he returned to New Zealand, publicly claiming that Fraser had double-crossed him over the Washington posting. He resigned from cabinet, but failed to get party support. In September 1943 the Evening Post alleged that Langstone had been recalled for serious misconduct. He took a libel action for £2,000 in damages, and was awarded £200 in February 1944.
Langstone continued to promote state control of the Bank of New Zealand, and at the 1944 Labour Party conference this received official endorsement. At the election of 1946 he was elected for Roskill, new boundaries for Waimarino involving too much strenuous travel. Although at the time he was regarded as a leading spirit among more radical Labour people, in 1947 he missed selection for cabinet by a wide margin and turned his attention to opposing New Zealand's joining the International Monetary Fund.
Peter Fraser's support for peacetime military conscription was the catalyst that finally led Langstone to resign from the Labour Party on 7 August 1949. He had exhaustively campaigned against conscription during the referendum on the issue. At the 1949 election he stood as an independent Labour candidate and was defeated. He remained active in politics, and in 1951 issued a pamphlet opposing that year's Waterfront Strike Emergency Regulations. He later joined the New Zealand Social Credit Political League and at the 1957 and 1960 elections stood for Roskill, stressing the need for a stable money supply and a financial credit authority to provide debt-free loans for public projects.
Agnes Langstone died on 5 August 1946, and on 11 January 1952 Frank married Catherine Mary (Mollie) Nolan at Auckland. He died of a heart attack at Auckland on 15 June 1969, survived by Mollie, and his three sons and two daughters.
Frank Langstone was a significant figure in the New Zealand Labour Party. As an orator, he rivalled Robert Semple and John A. Lee. He contributed to the formulation of Labour's policy in the 1920s and 1930s, and was one of the government's ablest administrators. However, his support for views not shared by more senior colleagues, especially his criticism of Labour's financial policies while in office, prevented his being appointed to more important positions.