David MacNish, whose surname was sometimes spelled McNish, was born probably in 1812 or 1813 in the parish of Trelawny, Cornwall, Jamaica. He was the son of David McNish, an estate overseer from Kirkcudbrightshire, Scotland, and Rebecca (Becky) Molloy, a slave of mixed British and African ancestry. David and his brother William were baptised on 23 February 1820, and manumitted from slavery by their father on 17 October that year. Both boys were educated at Gatehouse, Kirkcudbrightshire, and London. They were left substantial fortunes by their father, who died in 1827.
Exposed from an early age to prejudice because of his mixed-race parentage, David felt no binding ties to either Jamaica or Scotland. His adventuring spirit led him to India on a shooting holiday in the early 1830s. He then decided to make a new start in South Australia. He arrived in Adelaide in 1838 and spent two years there, acquiring land before moving on to New Zealand. MacNish arrived at the Bay of Islands on the Agenoria on 19 May 1840 and proceeded south to Whaingaroa (Raglan Harbour). There he entered into a common law marriage with Te Ani (later baptised Katerena), daughter of Te Moanaroa of Ngāti Tahinga.
According to one family tradition, the relationship began when David MacNish, a keen sportsman, beat the local champion wrestler in a Māori sports event. The loser's sister, Te Ani, admired the feat. 'Kapai te Pākehā' (The Pākehā is very good), she exclaimed, and a relationship developed between the pair. Another version is that the local people arranged MacNish's marriage to Te Ani in order to retain for the family the prestige and advantages that came from having a Pākehā resident among them. The couple settled with Ngāti Tahinga at Waikorea, between Whaingaroa and the mouth of the Waikato River; after 1845 they moved to Te Horea, on the northern side of Whaingaroa. David and Te Ani had seven children, all of whom went by the slightly altered surname, McNeish.
David MacNish became fluent in the Māori language. He was acting as government interpreter in Auckland in 1844. The explorer-artist G. F. Angas noted that, 'The Government interpreter is also married to a native woman, the daughter of Tepene, or Stephen, whose native name is Moanaroa…. She has proved herself an excellent wife, and has several children whom she keeps remarkably neat and clean, and sends daily to school in Auckland, where they receive an English education.' MacNish was an interpreter at the Raglan resident magistrate's court in 1860. In addition he worked as a labourer and bricklayer. In 1863 he cultivated land near the Raglan township as a 'sufferance' occupier according to the communal Māori custom.
In the late 1850s David MacNish's family in Galloway sent out a shipment of costly merchandise, but as the ship came via Sydney, it had to detour to New Plymouth to pay customs and excise duty before sailing to Raglan. MacNish objected to the added cost and told the captain to return the whole cargo to Scotland. Thus this early Pākehā-Māori settler continued to turn his back on his Scottish heritage and merged into the life of Ngāti Tahinga, among whom he died at Te Horea on 10 April 1863.