Thomas Brown (known as Broun) was born in Edinburgh, Scotland, on 15 July 1838, the son of John Brown, a soldier, and his wife, Margaret Stewart. Both his father and an uncle were naturalists of considerable repute. From an early age Broun took a keen interest in natural history, especially in all forms of insect life, and gardening. By the time of his death he was regarded as one of the world's leading authorities on beetles (Coleoptera).
Broun was educated in Edinburgh by a private tutor. During the Crimean War he served with the 35th (Royal Sussex) Regiment of Foot, which he then accompanied to Burma. The brilliant colours of many of the tropical insects attracted his attention, and he started a collection for the British Museum. The outbreak of the Indian war in May 1857 stopped this project when his regiment was dispatched to Calcutta. He served in India throughout the war and received the campaign medal. Near the end of 1861 he contracted cholera and almost died. He was invalided home in 1862 and retired from the army.
On 26 March 1863 Broun married Ann (Anne) Shepherd at Edinburgh; she was highly educated, a talented linguist and musician, and a lover of bird and animal life. They emigrated to New Zealand later in the year, and were to raise a family of at least six daughters. Broun had letters of introduction from the duke of Hamilton to Governor Sir George Grey, and on 19 September 1863 was offered a commission as captain in the 1st Regiment of the Waikato Militia. He served in South Auckland, Waikato and the Bay of Plenty and commanded a number of isolated redoubts. In 1916 he received the New Zealand War Medal after proving that he had been under fire during an engagement.
Broun left the army on 4 December 1866 with an entitlement to a Crown land grant, and took up farming at Opotiki. The venture was not successful. Allegations that he had withdrawn the money of four privates but had not paid it to them led to a refusal to issue his Crown grant. He was declared bankrupt in mid 1867, and the allegations were not disproved until the end of the year.
Broun continued to collect and describe insects, particularly beetles. He usually worked at night, at times enlisting the services of his daughters to sort out specimens from his samples. He presented the first of his many papers to the Auckland Institute in 1875. From 1876 to 1888 he was employed as a teacher at Tairua, Whangarei Heads, Kawau Island and Howick. During this period he worked hard on the first volume of his Manual of the New Zealand Coleoptera, which appeared in 1880 and contained descriptions of 1,140 species. Six further volumes were published, the last in 1893. Later descriptions were published in the papers of scientific societies.
As a result of this work, in 1894 Broun was appointed to the Department of Agriculture as government entomologist at Auckland. Between 1896 and 1907 he was also the inspector (latterly chief inspector) of imported fruit at Auckland. He lived at Karaka, near Drury, and following his retirement in 1907 shifted to Mount Eden. He died at Auckland on 24 August 1919, survived by his wife and six daughters. Ann Broun died in 1923.
Broun was an avid collector for over 50 years. He found 976 new species altogether, although the majority he described were sent from collectors throughout New Zealand. By the time his final paper was published posthumously in 1923 he had recorded and provided descriptions for 4,323 species; 3,538 of his nominal species were new to science. Only 37 were illustrated, meaning most species could not be recognised without having the original specimens available. Unfortunately, Broun bequeathed his principal collection to the British Museum. It was sent there in 1922, despite its export having been banned at the instigation of leading New Zealand scientists. Other collections, including one of over 1,000 species of exotic beetles from Madagascar and Africa, are now held by the Auckland Institute and Museum and Landcare Research, Auckland. Although many of Broun's descriptions of New Zealand Coleoptera are now obsolete, he had made perhaps the most important individual contribution to identifying and describing New Zealand's beetles.