Thomas Hall was born at Kingston upon Hull, Yorkshire, England, probably in 1847 or 1848, the son of Thomas Williamson Hall, a farmer, and his wife, Sarah Young. The family arrived in Timaru, New Zealand, about 1865 and as a youth Thomas Hall managed a sheep station owned by his uncle, John Hall, later to be premier of New Zealand.
Hall's family was prominent in Timaru, and Hall himself was in the 1880s one of the town's most eligible bachelors. He was 'good-looking, dark, slightly built and of medium height.' 'He talked well, dressed well, shot well, danced well, was ready for any amusing enterprise, always had a good horse and knew how to sit it.' Many of Timaru's young ladies boasted of having an 'understanding' with him; they were later to deny ever having known him.
In the early 1880s Hall became a partner in Hall and Meason, commission agents, selling land and lending money. He married Kate Emily Espie in Timaru on 26 May 1885. She was a stepdaughter of the reputedly wealthy Captain Henry Cain, one of Timaru's earliest residents and a former mayor of the town. Her mother had died in 1878. On 24 July 1885 Thomas gave instructions for the drawing up of Kate's will; it bequeathed to her husband all her real and personal property. In August of the same year he took out two life insurance policies in her name: one was for £3,000 payable on her death; the other, for the same amount, was payable if she died in seven years.
Henry Cain died at his home, Woodlands, on 29 January 1886. He had been suffering from an unspecified illness during which his son-in-law was in constant attendance on him. Cain frequently complained that the whisky provided by Hall was the cause of his illness. Soon after his death Thomas and Kate Hall, together with a live-in companion, Margaret Houston, moved into Woodlands.
The Halls' only child, Nigel, was born on 19 June 1886. Kate Hall fell ill shortly after the birth, exhibiting symptoms which puzzled Patrick McIntyre, the family doctor. Even more puzzling was the fact that they appeared to worsen when her medicine was administered by her husband. When a visiting family friend inadvertently drank tea from a cup prepared for Kate and herself fell ill, rumours of poisoning began to spread. Dr McIntyre, at last realising that his patient's symptoms were consistent with antimony poisoning, had a sample of her stomach contents sent to Dunedin for analysis. His suspicions were confirmed and on 15 August 1886 Thomas Hall and Margaret Houston were arrested on a charge of attempted murder. Hall also faced 12 charges of forgery. Hall and Meason quickly went into bankruptcy, and it was found that the firm's two accounts, operated by Hall, had been overdrawn for some time.
After a depositions hearing at Timaru attended by large crowds, Hall and Houston were committed for trial in Christchurch. The evidence against Hall was overwhelming: his financial affairs and the potential for profit from Kate Hall's will and insurance policies provided a clear motive. He was known to have purchased antimony and two books on poisons, and antimony was found in his pockets following his arrest; he had then, it was said, told Margaret Houston that he could not possibly escape conviction. The evidence against Houston was circumstantial: she had helped administer medicine to Kate Hall, and family servants claimed that she and Hall had engaged in undue intimacy. She was acquitted; Hall was sentenced to life imprisonment. The presiding judge, Alexander James Johnston, called him 'the vilest criminal ever tried in New Zealand.'
Dr McIntyre, recalling Henry Cain's curious illness, had had the body exhumed; it, too, showed unmistakable signs of antimony poisoning. On 25 November 1886 Hall was charged with the murder of his father-in-law. After a trial at Dunedin, he was found guilty and sentenced to death by the judge, Joshua Strange Williams. He appealed, and the conviction was overturned on a technicality. The news caused outrage in some quarters: it was claimed that 'had hall been an ordinary member of the community, related in no way to the "upper ten", it is in the last degree improbable that he would now be alive.'
Hall was released from Mount Eden prison, Auckland, in 1907. Sir John Hall granted him an annuity of £200 on condition that he live the remainder of his life out of New Zealand. Kate Hall had secured a divorce in 1902; she and her son had been living in England for some years. Thomas Hall may also have gone there or to Australia. He is said to have constantly pestered the trustees of his annuity fund to be allowed to return to New Zealand. He reputedly supplemented his income by selling photographs. The date and place of his death are not known.