William Hall Jones was born on 16 January 1851 at Folkestone, Kent, England, the son of William Hall Jones and his wife, Margaret Hall. His father, a cabinet-maker, undertook the education of his children himself. At his father's wish William began his career as a clerk in the Rothschild financial firm, but desiring a more practical type of occupation he changed to carpentry.
In 1871 William moved to London where he worked as a carpenter. On 27 October 1873 he married Fanny Smith at Clerkenwell. The couple decided to emigrate to New Zealand where William saw the prospect of work as a builder. William and Fanny landed at Port Chalmers, probably in April 1874, and he began work as a carpenter in Dunedin. The couple later moved, first to Ōamaru, then to Timaru where, sadly, Fanny died of cancer on 10 January 1876. William worked for a building firm for six years before starting out on his own account. On 10 May 1877 at Timaru he married Rosalind Lucy Purss of Chertsey, England; they were to have a family of four daughters and two sons. William purchased a rural section at Scarborough where he built a permanent home for his growing family.
He gained his first experience in public life as a member of the Timaru Borough Council from 1884 to 1886, and again from 1890 to 1892. He also served on the Levels Road Board from 1887 to 1889. About this time he began to use the surname Hall-Jones. His activities on both bodies led to his being asked to become a candidate for Parliament at the Timaru by-election caused by the death of Richard Turnbull. Against a formidable opponent, Edward Kerr, he was elected member for Timaru at the by-election on 18 August 1890, a seat he retained until his resignation on 29 October 1908.
In Parliament Hall-Jones revealed himself to be an independent thinker who generally favoured Liberal policies, and he was invited to join the Liberal party of John Ballance. After the success of Ballance's party at the general election in 1890 he was asked to become government whip. However, two of his main election platforms had been the subdivision of large runs into farm settlements and women's franchise, and after his party's inaction in these two fields he resigned his post as government whip in 1893 and became an independent member. From 1893 to 1896 he sat almost alone in the House, speaking as an independent but always supporting the Liberal government of Richard Seddon in a no-confidence motion. He so impressed Seddon with his carefully thought-out arguments, his candour and his abstention from personalities that in February 1896, when William Pember Reeves resigned to become agent general for New Zealand in the United Kingdom, Hall-Jones was invited to fill the cabinet vacancy as minister of justice. He accepted, but Joseph Ward resigned as minister of marine soon after and Hall-Jones took on this portfolio instead, a position he held for the next 10 years. He served as minister for public works (1896–1908), minister for railways (1906–8), and colonial treasurer, minister of labour and minister of education in 1906.
As minister for public works Hall-Jones's major task was to oversee the completion of the North Island main trunk railway. Unhappy with the proposal for another Remutaka incline he insisted on a better route, which resulted in the Raurimu spiral. In the South Island he was responsible for the eventual construction of the Ōtira tunnel through Arthur's Pass. As minister of marine he came into conflict with the governor, Lord Ranfurly, when the latter attempted to extend his cruise of the subantarctic islands in the government steamer, Tutanekai, to the West Coast sounds. With supplies urgently required for the southern lighthouses Hall-Jones refused to grant permission for the extension. When the governor persisted Hall-Jones threatened to resign. His excellency, probably realising that he was on delicate ground, finally withdrew his request. Seddon was deeply impressed by the correctness and firmness of his minister and thereafter Hall-Jones became one of the most influential members of the cabinet. In 1898 he had the satisfaction of seeing the Old-age Pensions Act finally passed. His idea of 'non-contributory' pensions paid out of the Consolidated Fund, a scheme that he had advocated himself in his independent days, had finally been accepted by his party; it was a pioneering piece of welfare legislation in New Zealand. Other policies which he had advocated strongly also came to fruition: farm settlements on large estates and women's franchise.
In 1906 when Seddon decided to visit Australia he asked Hall-Jones to serve as acting premier, but he demurred. Ward, the senior minister, was away at a postal conference in Rome and Hall-Jones felt that James Carroll, the next in line, should have the honour. Seddon persisted, and after being assured that Carroll was in agreement Hall-Jones accepted the position. On 10 June 1906 Seddon died at sea while returning from Australia. The governor, Lord Plunket, urged the acting premier to form a ministry forthwith. Although Hall-Jones agreed to do so in due course, he refused to act until after his leader and personal friend had been buried. The governor replied that whereas the delay was without precedent his wishes would be respected. He also communicated the reason for the delay to the Colonial Office in London. And so it was not until 21 June 1906, 11 days after Seddon's death, that Hall-Jones was sworn in as prime minister, the first to be officially so designated. He retained his old portfolios in the new ministry, adding those of finance, education and labour. The ministers remained as before with no change in portfolios. There was no controversial legislation during the following session and on Ward's return from overseas Hall-Jones courteously offered to resign in his favour, an offer that was accepted by Ward without hesitation. Hall-Jones resigned on 6 August 1906, after 6½ weeks in office, but agreed to carry on in the Ward administration with his old portfolios of railways and public works.
In 1907 Hall-Jones was again serving as acting prime minister while Ward was overseas. Later that year he suffered a severe heart attack after which he was granted six months' leave to take a long sea voyage to England. In 1908 Reeves, whose position had been designated as high commissioner since 1905, intimated his wish to retire. On 30 November 1908 Hall-Jones resigned from cabinet and on the following day it was announced that he would succeed Reeves. He sailed for England with his wife that December.
In 1910 Hall-Jones represented New Zealand at the second International Congress of Refrigeration in Vienna where he was influential in persuading delegates to adopt a resolution of protest against the almost prohibitive restrictions on the entry of frozen meat. Arranging a private meeting with representatives from Australia and Great Britain he suggested a resolution in favour of fewer restrictions and was asked to move it. The resolution was passed unanimously and has had far-reaching effects for New Zealand. Later he regarded this as the most important diplomatic success of his whole political life. He also arranged for a New Zealand pavilion at the Roubaix exhibition in France where a range of first-class exhibits of frozen meat was displayed, the first shown on the Continent. At various times Hall-Jones represented New Zealand on the Pacific Cable Board, the Imperial Wireless Committee and the Imperial Copyright Committee.
Hall-Jones was appointed a KCMG in 1910. It was a rather unusual honour in that it emanated from the Colonial Office and not his own government, perhaps a reflection of the respect that he had gained among British politicians. Sir William retired as high commissioner on 31 May 1912.
On his return to New Zealand in 1913 Prime Minister William Massey, the leader of Hall-Jones's political opponents, invited him to become a member of the Legislative Council. He accepted and remained a member until his death in 1936. In 1928 he revisited Great Britain where he was presented with the freedom of Folkestone, the highest honour that his native city could bestow. He died at his home at 6 Burnell Avenue, Thorndon, Wellington, on 19 June 1936 and was buried at Karori cemetery. He was survived by his wife, who died in Wellington on 13 October 1942 and was buried alongside him.
Never self-seeking nor assertive, Hall-Jones was liked and trusted by politicians on both sides of the House and by people in all walks of life. He gained a special reputation as a policy maker and an administrator. It was his old chief Richard Seddon who once said of him, 'he is the best administrator I have in my Cabinet'.