Thomas George Macarthy was born in London, England, probably in 1833 or 1834, the son of Thomas Bernard Macarthy, a florist, and his wife, Ann Elizabeth Grout. Nothing is known about his early life. In the 1850s he went to the goldfields in Victoria, Australia, and in the 1860s, Otago, New Zealand. On the West Coast he acquired a brewery in Charleston, and reputedly had lucrative investments in the Reefton area.
In 1877 Macarthy moved to Wellington, where he purchased a brewery in Old Custom House Street (Bond Street) and in 1883 a larger brewery in Tory Street. He also acquired hotels which were closely linked to his principal business, and other urban property. The financial liquidity of brewing made it a sound investment during the depression of the 1880s and early 1890s.
Macarthy became a respected member of the Wellington business community. He was prominent in the Wellington and Manawatū Railway Company, the Wellington industrial exhibition of 1896 to 1897, and the company formed in the 1890s to build an opera house. He was elected to the board of directors of the Bank of New Zealand in 1894 when the location of its head office was shifted from London to Wellington, and served until 1898. He was also a member of the Wellington Racing Club.
On 28 April 1897, when aged 63, Macarthy married 23-year-old Mary Ellen Fitzsimons at St Mary of the Angels Catholic Church in Wellington. There were no children of the marriage. He remained active in management until his final illness, and died in Wellington on 19 August 1912. After providing for personal bequests, Macarthy left half the residual estate to his wife, and half for the purpose of establishing a trust for 'charitable and educational purposes or institutions in the Provincial District of Wellington'. The trust was also to inherit from his wife if she had no heirs, and this was the case. Mary Macarthy married Walter Edward Reid, a merchant, on 21 January 1914, and the marriage was childless. She died on 1 July 1934. Thomas Macarthy's estate was estimated at £369,689, which put it well within the top 0.1 per cent of all New Zealand estates. The Thomas George Macarthy Trust distributed an average of just under £10,000 per annum in its first 20 years.
Thomas Macarthy was a quiet and retiring man. Although Mary Macarthy was involved in charitable activity and Thomas gave to charities anonymously, little of this was known during his lifetime. The size and prominence of the Macarthy Trust made his name well known: governors of the trust included the prime minister, the Catholic archbishop of Wellington, the governor, and the mayor of Wellington. All consented to serve and Parliament acted immediately to give the trust corporate status and to clear away any legal doubts over the perpetual nature of the trust when Macarthy's widow died. The public trustee administered the trust and by 1989 over $10 million had been distributed. The trust attracted attention in the late 1920s when prohibition was a significant political issue – the leading prohibitionist Sir Robert Stout attacked the role of the public trustee in continuing to run Macarthy's brewing business.