The death of Richard Turnbull triggered a by-election in the Timaru electorate, which was won by Hall-Jones on 18 August 1890. Hall-Jones had initially refused nomination from locals, citing several upcoming business contracts. However, after persistent calls, Hall-Jones reluctantly accepted despite having no parliamentary ambitions. He represented Timaru in the House of Representatives until his resignation in October 1908. Hall-Jones proved an independent thinker. He was initially an Independent Liberal holding moderate, progressive views that tended to align him with John Ballance, Sir George Grey and John McKenzie. He joined the Liberal caucus and in 1891 became the party whip alongside Westby Perceval.
Cabinet Minister
Hall-Jones became a cabinet minister in March 1896 given the Public Works portfolio by Prime Minister Richard Seddon following William Pember Reeves resignation to become Agent General for New Zealand in the United Kingdom. His main task in this role was improving the main trunk rail line between Auckland and Wellington. Rejecting a proposal for another incline on the Rimutaka ranges he insisted on using a better route, resulting in the Raurimu spiral. He was also responsible for the eventual construction of the Otira tunnel, going through Arthur's Pass. Several weeks after entering cabinet Hall-Jones was also appointed Minister of Marine, which he was to hold for over a decade. Hall-Jones was also responsible for passing a bill granting protection to famous navigation dolphin Pelorus Jack by Order in Council under the SeaFisheries Act on 26 September 1904.
Prime Minister
Hall-Jones was acting Prime Minister during the absence from the country of Seddon in 1906 and formed an administration immediately after Seddon's funeral. During his brief period as Prime Minister, he was Colonial Treasurer, Minister of Labour, Minister of Education, Minister for Public Works, and Minister of Marine. However, Hall-Jones announced that he would only hold power until Sir Joseph Ward's return from abroad. Despite this, there was much speculation in the media that he may attempt to remain in office as Seddon himself had done in 1893.
Later career
Hall-Jones accepted the Railways and Public Works portfolios in the subsequent Ward administration. Later, he succeeded William Pember Reeves as High Commissioner for New Zealand in London in December 1908, returned to New Zealand at the end of his term in 1912, and was appointed to the Legislative Council by Massey. Hall-Jones died at his home in Wellington on 19 June 1936.