Thomas Whitelock Kempthorne was born in Cornwall, England, the son of John Kempthorne, a builder, and his wife, Anne Whitelock. He was baptised in the parish of Mawnan, near Falmouth, on 3 February 1834. In 1854 he emigrated to Melbourne, Australia. He worked briefly as a labourer digging up tree stumps, and resisted the temptation to join the Victorian goldrushes. In January 1855 he found a position in the wholesale drug trade, and rose to manage a druggist's business owned by Benson Brothers. On 23 June 1859 in Melbourne he married Sarah Inman, formerly Moody, a widow with two children. They had five daughters and one son before Sarah's death in 1895.
Kempthorne arrived in Dunedin in April 1863, sent by the Melbourne pharmaceutical firm of H. & E. Youngman to explore the prospects of establishing a branch there. When the company's principals died Kempthorne remained in Dunedin and set up his own wholesale drug business. In 1870 Kempthorne, Prosser & Company was formed when he went into partnership with Evan Prosser, a Welsh chemist who had been in business in Queenstown and on the West Coast. The partnership prospered and in 1879 they set up a limited liability company with nominal capital of £200,000. A head office building, designed by F. W. Petre, was erected in Dunedin.
Kempthorne, Prosser & Company's New Zealand Drug Company Limited imported drugs and medical supplies, and also manufactured pills and patent medicines. Warehouses were established in Auckland, Wellington, Christchurch and Dunedin. In 1881, responding to the incentive of a government bonus, the company established New Zealand's first acid manufacturing plant at Burnside, near Dunedin. From this stemmed a major chemical fertiliser industry, of national importance to the agricultural and pastoral economy. For almost a century 'K. P.' was to dominate the market. Prosser resigned from the company in 1886 and went into business in Australia.
In business Kempthorne was both dynamic and meticulous. He became a prominent figure in the Dunedin business community, remaining managing director of Kempthorne, Prosser & Company until 1904. He was a director of the National Insurance Company, the Perpetual Trustees Estate and Agency Company, and Donaghy's Rope and Twine Company Limited.
He expressed his philanthropic inclinations by generous support of St Paul's Cathedral in Dunedin, and in 1893 was one of the promoters of a scheme to reduce unemployment by putting men to work improving city reserves. His public positions were few. He was a member of the board of managers of the Dunedin Technical School (later King Edward Technical College) from 1911 to 1915 and made a donation of £900 towards its new building. He also served as chairman of the organising committee which equipped the Otago contingent to the Boer War. On 29 March 1899 in Dunedin he married Annie Charlotte Rawdon Robinson, about 40 years his junior. They had one daughter. Annie Kempthorne died in 1903 aged 31 years.
In prosperity Kempthorne displayed a grand manner. He affected a top hat and a flowing cloak and listed himself as 'gentleman'. In 1901 as the first Dunedin citizen to own a car he created a minor sensation when he was driven along Princes Street in his steam-operated two cylinder Locomobile with one of his company's engineers at the tiller. In 1909 he published A trip to Australia and other Pacific islands, containing his observations and ruminations on a journey which he had made with two of his daughters. New Zealand, he concluded, was 'a land…in which man indeed is blest.' He became ill at a board meeting of the National Insurance Company and died of a cerebral haemorrhage in Dunedin on 3 November 1915.