Thomas Tanner was born in Wiltshire, England, probably in 1830, and was baptised at Devizes on 31 July 1831. He was the son of Mary Pontin and her husband, Joseph Tanner, a landowner. After studying medicine briefly, Thomas Tanner came to New Zealand in 1849 on the Larkins and worked as a cadet on a Whanganui sheep run. There he acquired a knowledge of Māori culture and a Māori version of his name, Tanera.
In 1853 Tanner took up a large run, Milbourne, on the Ruataniwha plains, Hawke's Bay. He promoted horse-racing, and selected the course at Clive for Hawke's Bay's first formal race meeting in 1856. After a few years he returned to England and married Julia Denton at Hartlepool, Durham, on 5 May 1859. There were to be eight children of the marriage. The couple came out to New Zealand about 1862 on a chartered ship with furniture, books, servants and pedigree animals.
Tanner took an important step in 1864 when he obtained an illegal lease on the Heretaunga block, then a swamp but today one of the most valuable pieces of land in New Zealand. He divided the land into twelve shares, and, retaining four for himself, distributed the rest among six of his associates: J. N. Williams, A. H. Russell, T. E. Gordon, J. D. Ormond, T. P. Russell and J. B. Braithwaite. These men became known as The Apostles. In 1867 a Crown grant was issued on Heretaunga in the name of 10 Māori owners and Tanner obtained a legal lease.
In the next few years Tanner obtained freehold title for the Heretaunga block, in the face of opposition from some Māori owners and from Europeans who deplored the control of large areas of land by a few. Using persuasion and sometimes force, he pressured the grantees until one by one, they sold. By 1870 Heretaunga was freeholded. Tanner also bought land on the Ahuriri plains, and the Endesleigh and Pētane runs.
Tanner became established as a dominant figure in Hawke's Bay affairs. He named his 5,332 acre portion of the Heretaunga block Riverslea, and built a 22-roomed mansion there. A landscape gardener was brought from England to lay out the grounds and an architect designed all the buildings, including the stables. The Tanners proceeded to show Hawke's Bay what gracious living was all about. Only one other family could boast gold plate on the table.
When war spread to the East Coast in 1868 and 1869 Tanner raised his own fighting force, the Hawke's Bay and Waipawa Yeoman Cavalry. He was commissioned captain and supplied his men with uniforms, mounts and arms, hiring the Oddfellows Hall in Napier as his private drill hall. The cavalry saw action at Gisborne in 1868 and at Mōhaka in 1869.
Tanner was not simply a land speculator: he was committed to the idea of scientific farming. In order to drain and clear his Riverslea estate he needed both labour and capital. First he leased portions of his land and after obtaining the freehold, in 1871 he advertised for sale sections at Karamū. This area became closely settled. After 1873 small farmers subdivided their land into town sections and from the settlement of Karamū grew the town of Hastings. Tanner later called himself the town's father, because he owned the land in the first place, and its godfather, because he chose its name. He bought a section for an Anglican church and he gave land for a school and a public park. He also set aside two sections for a municipal building, for which he guaranteed building costs.
In 1873 the Heretaunga purchase was the subject of a commission of inquiry presided over by C. W. Richmond and F. E. Maning. There was an outcry: Tanner and The Apostles were attacked as instigators of grog mortgages and forced sales. However, fraud charges were not proved. Tanner survived the scandal because of his character and his ability as a businessman. During the inquiry Richmond wrote in his journal, 'I must say I like Tanera: He is no doubt a self confident little man – some might say conceited, but I don't give it that name: he is thoroughly self reliant & avows it'. Settlers in general did not like him but they eagerly followed his lead. Māori continued to co-operate with him even when he was found to have manipulated them because he understood their customs and always worked through the chiefs.
By the late 1870s Tanner's fortunes were on the wane. He had borrowed at high interest to purchase land and, as economic depression deepened, he was forced to subdivide and sell large areas of the Riverslea estate, in 1879, 1885 and 1889. He turned his attention instead to schemes for developing industries. This interest was not new. When leading the Hawke's Bay cavalry into action, he had been given a piece of coal picked up by a trooper at a stream crossing. After the emergency was over he had sent a coal prospector back to the area. He was one of the directors of the Hawke's Bay Goldmining Company which unsuccessfully prospected near Taupō in 1869, and was later, in 1880, a leading investor in the Mōhaka Goldmining Company.
Tanner searched for ways of enriching himself and, at the same time, providing opportunities for small farmers. During the 1870s and 1880s he tried sugar-beet and tobacco growing and failed. In 1883 he invested heavily in hop growing and set up a processing plant at Riverslea as an outlet for small hop growers. However, his attempts to ship hops failed and he lost heavily. He set up a syndicate to promote a woollen mill in Hastings in 1887 and gave 6½ acres of land for the project, which did not eventuate. Although these schemes were largely unsuccessful, they illustrated Tanner's energy and his confidence in the potential of Hawke's Bay.
His sense of commitment was reflected also in his involvement in public affairs. He helped found the Hawke's Bay Agricultural and Pastoral Association in 1873 and was a long-standing member of the Hawke's Bay Philosophical Institute, the Hawke's Bay Education Board, and numerous other organisations concerned with cultural and social life. A devoted member of the Church of England and a friend of Bishop G. A. Selwyn, he was present at the signing of the Constitution of the Church of the Province of New Zealand. He was the principal benefactor of St Luke's Church in Havelock North, and its vicar's warden and synodsman. Tanner signed the contract and contributed financially to the building of the church. Many of the trees of Hastings and Havelock North were planted by him and the pin oak outside St Luke's Church is known as Tanner's Oak.
Tanner was at various times a member of the Heretaunga Road Board, the Hastings Town Board and the Hastings Borough Council, and was a Hawke's Bay county councillor between 1878 and 1893. He served on the Hawke's Bay Provincial Council from 1867 until 1876 and was member of the House of Representatives for Waipawa from 1887 to 1890. He died at Havelock North on 22 July 1918.