Thomas Fleming, the third child of Thomas Fleming, a farmer, and his wife, Janet Scoular, was born at Holme Farm, Stonehouse, Lanarkshire, Scotland, on 5 April 1848. He was educated at the Glassford parish school. He came to New Zealand with his parents, four brothers and three sisters on the Storm Cloud, arriving at Campbelltown (Bluff) on 8 December 1862. Another sister was born a few days after the family landed.
Thomas, his father and older brothers soon set to work at various jobs in Invercargill. Thomas worked first as cow-boy and gardener for Peter Dalrymple, at 10s. a week. Then he and his father worked for Gravel and Whiting doing cartage work, for which Thomas was paid 8s. a day.
In 1864 Thomas's father bought 100 acres of land at Mabel Bush for 25s. an acre. While the land was being cleared, Thomas and his brothers remained at work in Invercargill. After driving the first mob of cattle to Mabel Bush, Thomas worked with his father for some years, both there and at the larger property, Rakahouka, purchased in 1866.
In 1870 Thomas Fleming visited his brother Andrew, who was teaching at the Oamaru Grammar School, and was soon employed harvesting wheat on the New Zealand and Australian Land Company's station at Totara. Later the same year he was persuaded by Andrew to begin learning the flourmilling business at the company's Kakanui mill. He became its manager in 1872 or 1873.
Fleming moved south about 1875 to manage the new flour mill built at Invercargill by John Murdoch. He purchased it, probably in 1876, and took in a brother-in-law, Peter Gilkison, as partner. They competed successfully with the millers of North Otago and Canterbury, and soon began taking over smaller mills around Southland.
In April 1889 the wooden Invercargill mill was destroyed by fire. The firm continued fulfilling contracts by using its other mills at Gore, Winton and Mataura while a new four-storeyed brick mill was built, lit by electricity generated on the premises, and with a modern roller flourmilling plant. The flour produced there was whiter and finer than the former stone-ground flour, and the mill was also capable of much larger production.
Fleming and Gilkison continued to take over smaller mills, for instance at Riverton and Otautau; these were gradually shut down and flour production centralised at the Invercargill mill, which eventually became the only flourmilling plant in Southland. The oatmeal plant was transposed to the Gore mill, which was also rebuilt and modernised in 1893 so that it was capable of much larger production. In 1902 Fleming bought out the interests of his former partners and converted Fleming and Gilkison into Fleming and Company Limited. John Rennie, his son-in-law, became a partner in the reconstituted firm.
Fleming seems to have been an enthusiast for modern technology. He always took a great interest in selecting the latest plant for his mills, and between 1886 and 1910 made five visits to England partly for this purpose. In 1895 he applied unsuccessfully to the Invercargill Borough Council for permission to run a line from the electricity generator at his mill to his gas-lit house. He is also said to have owned the first motor car in Invercargill.
Fleming married Marion Pollock at Otepopo on 17 March 1875; there were four daughters and three sons of the marriage. Two of the sons, Herbert and Andrew, were to make their careers in the firm; a third, Thomas, qualified as a doctor at the University of Glasgow and returned to practise in Gore and Mataura. After Marion Fleming's death in 1905, Fleming married Ellen Freeman at Green Island on 7 March 1906.
Fleming became a well-known businessman and citizen. He served on the Invercargill Borough Council, and as mayor from 1888 to 1889. In 1911 he stood for Parliament as Reform candidate for Invercargill. Although he was defeated his campaign was widely admired, and the Dominion praised him as 'one of the soundest and shrewdest of the 270 odd candidates now before the country.…with a finish and aplomb that even the ablest and most experienced members of the Reform Party will not be likely to surpass.'
In 1912 Fleming retired and sold his shares in the firm to Herbert and Andrew, and John Rennie. He moved to Dunedin where he lived quietly, devoting much of his energy to gardening. In 1925 or 1926 he moved to Timaru, where he died on 14 October 1930, survived by Ellen Fleming and their two sons, as well as four daughters and a son from his first marriage.