James Thorn was born in Christchurch on 1 June 1882, the son of John Thorn, a master butcher, and his wife, Jessie Alston. John Thorn had apparently possessed modest means, including racehorses, but lost much of his property in the banking crash of 1893.
Jim Thorn showed early promise. He won a scholarship to Christchurch Boys' High School, and three years later was offered a position as articled clerk by a solicitor who worshipped at the same church as the Thorns. However, John Thorn blocked his son's enthusiasm for this offer and had him apprenticed as a moulder.
In 1900 Jim Thorn was ordered by his father to enlist for military service in South Africa. While this offered some relief from the moulding trade, witnessing the killing of civilians turned him into a convinced pacifist. On his return from South Africa, his father's connections secured Thorn a job in the Addington Railway Workshops. At this time he also became a Presbyterian lay preacher. Researching for a Bible class debate on socialism, Thorn talked to Jack McCullough, the workshops' greatly respected advocate of independent labour politics, and was thoroughly converted. Rising quickly in labour organisations, by 1905 Thorn was secretary of the metalworkers' union and correspondence secretary for the powerful Canterbury Trades and Labour Council.
In 1904 the Political Labour League of New Zealand – the country's first independent labour party – was formed. Jim Thorn was selected for Christchurch South in the election at the end of the following year. Despite standing against Harry Ell, one of the more left-wing Liberals, he polled 1,107 votes, far more than any of the league's 10 other candidates. His speeches drew attention (and criticism) for their vigour; he accused the Seddon government of 'propitiating Conservatives in the matter of Crown lands, catering for wool kings and lawyers'.
After the 1905 election Thorn, with Jack McCullough, Edward Kennedy, Pat D'Arcy and Ettie Rout, put much effort into organising Canterbury's agricultural labourers. Kennedy and D'Arcy had worked in the countryside, Rout ran the office in town, and McCullough and Thorn were involved through the trades council. Being less vulnerable than rural workers to victimisation, McCullough and Thorn served respectively as president and secretary of the union. After putting in a full day at the railway workshops they would often cycle 30 or 40 miles, conduct a meeting, return home, and catch a few hours' sleep before work the next day. During 1907 the union membership grew to 1,500. Their claims – for regulated wages, accommodation, and some provision for holidays – were relatively modest. However, after months of hearings, including an eight-day closing address by Thorn (brevity was never one of his virtues), the labourers' case was turned down by the Court of Arbitration in August 1908. With all its money spent the union virtually collapsed. In the general election shortly afterwards, Thorn again stood in Christchurch South and polled well. The Political Labour League dissolved soon after the election, weakened by debt and disputes, and Thorn left the country in 1909.
Thorn's overseas trip was sponsored at least in part by friends and associates as a way of broadening his mind and furthering his political education. Rout and McCullough saw him as one of the labour movement's greatest hopes. Thorn acknowledged these two, and Christchurch's radical Liberal politician, Tommy Taylor, as his principal mentors. As late as the 1950s Thorn was still voting for prohibition, in deference to Taylor's views.
Jim Thorn spent four years in Britain as a socialist propagandist and journalist associated with the Clarion and Forward newspapers, which propounded the radical and ethical socialism and anti-militarism that had become Thorn's own. He contributed occasional articles to the Maoriland Worker, generally contrasting imperial rhetoric with the brutal realities of British foreign policy abroad and the lot of the poor in British cities. By the time Thorn returned to New Zealand – the day the great wharf strike of 1913 began – he had abandoned his religious beliefs but not his moral fervour. He served on the Christchurch strike committee in 1913, and was then appointed a roving organiser for the new Social Democratic Party.
Early photographs suggest a very good-looking young man; to his looks Thorn added a dynamic personality and eloquent idealism. He had a succession of love affairs in New Zealand and England. An engagement in England was ended by the young woman concerned, who wanted someone slightly less utopian in his views; another in New Zealand was terminated by the prospective mother-in-law, who objected to Thorn's politics. Thorn also cherished an apparently unrequited passion for Ettie Rout, five years older, and at least his equal when it came to espousing romantic socialism.
During 1914 Thorn moved to Palmerston North, where he stood unsuccessfully for Parliament at the end of the year. He briefly grew tomatoes for a living before taking up a position with the Maoriland Worker at the beginning of 1916. At the end of that year he, along with other leading Labour figures, was arrested and imprisoned for 12 months for speaking against conscription. On his release from Mount Eden prison, Thorn married Margaret Anderson at Wellington on 8 December 1917; they had two daughters and a son. Margaret had been born in Lancashire and had been active in trade union and socialist affairs from her teens; she continued this involvement after their marriage. Possibly the timing of the wedding was an attempt to avoid conscription, for Thorn was served with call-up papers at the same time. Along with those of other 'defaulters', his civil rights were removed by executive decree in 1918; this extra-judicial proceeding dashed his immediate hopes of a political career. Like most victims, he suffered disenfranchisement for 10 years.
Denied a parliamentary career, Thorn spent most of the 1920s as editor of the Maoriland Worker (later the New Zealand Worker), and was secretary, vice president and president of the New Zealand Labour Party.
Thorn finally won election to Parliament in Labour's victory year of 1935, as member for Thames, a farming and goldmining electorate. His speeches were generally able statements of Labour's case, distinguished by a strong ethical cast. On one occasion he defended Labour's progressive taxation policy by saying that 'it is good for the soul of the people to be obliged by law to remember that they are their brothers' keepers'.
As a local MP Thorn urged agricultural development against the closure of the district's remaining goldmines. He also took a particular interest in commercial fisheries, which were a significant industry in Thames. He chaired a committee on commercial fisheries in 1937, and in 1938 went to Geneva as a delegate to the International Labour Organisation conference. From 1943 to 1946 he was parliamentary under-secretary to the prime minister.
With so many other Labour veterans in Parliament well ahead of him, Thorn had no hope of cabinet, and his 11 years in the house were perhaps marked by a lingering sense of disappointment. Few subjects seem to have really engaged him, with the exception of collective security and the United Nations. In 1938 and 1939 Thorn was vocal in his condemnation of the 'sinister confabulation at Munich', which had handed Czechoslovakia over to Hitler, and in his championing of an independent foreign policy for New Zealand. He saw the signing of the United Nations charter in 1945 as offering a real possibility of co-operative international development.
It was fitting, therefore, that when he lost his parliamentary seat in 1946, Thorn was appointed high commissioner to Canada, a job that also entailed representing New Zealand at the United Nations. He took particular interest in the Economic and Social Council. He also sat on the committee that drafted the Universal Declaration of Human Rights in 1948. Like other Labour politicians, Thorn believed that 'In a world deeply divided against itself the principles of the [United Nations] Charter provide a bridge which still swings perilously across the gulf.'
The last few years of Thorn's life were spent in retirement in Lower Hutt. He was president of the United Nations Association of New Zealand and wrote a number of pieces on New Zealand labour history, including a hagiography of Peter Fraser. He had chaired the National Centennial Historical Committee from 1937 to 1940. He died on 21 November 1956. Margaret Thorn remained active in socialist causes until her death in 1969.