Thomas William Potter was born in Streatham, Surrey, England, on 2 August 1843, the son of John Potter, an agricultural labourer, and his wife, Jane Phipps. He later changed his name to Porter and claimed to have been born in India, the son of John William Porter, an officer in the 7th Bengal Native Infantry, and his wife, Jane Emily Rose. (Thomas sometimes incorporated 'Rose' into his name.) Details of his early life are unclear. Porter claimed that he served as a midshipman in the Royal Navy between 1857 and 1859 and saw active service off the Chinese coast in 1858, and that between 1860 and 1863 he was attached to the 70th (Surrey) Regiment of Foot in New Zealand.
Between 1863 and 1866 he served in the Colonial Defence Force cavalry in Hawke's Bay under the name Potter. During this period he began to study the Māori language. He remained on the east coast after leaving the cavalry, and, on 9 May 1873 at Gisborne, married Herewaka Porourangi Pōtae, the daughter of Ngāti Porou leader Tama-i-whakanehua-i-te-rangi; they were to have at least eight children, among them the tattooer Tāme Poata and the singer Fanny Rose Porter, known as Princess Te Rangi Pai.
In 1868 Porter served with the Ngāti Porou contingent fighting Te Kooti. He performed well enough to be commissioned as an acting sub-inspector in the Armed Constabulary on 5 January 1869, and given the post of second in command of its No 8 (Arawa) Division. Porter's division took part in the final campaign against Titokowaru in Taranaki between January and April 1869, and in May formed part of the force which invaded the rough and inaccessible Urewera country. In July 1869 Porter resigned his commission and returned to the East Coast. There, on 31 August, he joined the Poverty Bay Mounted Rifle Volunteers, and in October was commissioned as a lieutenant in the corps.
Early in 1870 Major Rāpata Wahawaha named his friend Porter as the European officer he would like to accompany his force in its expedition into the Urewera mountains in pursuit of Te Kooti. Porter was commissioned as a captain in the New Zealand Militia in June 1870 and appointed adjutant of the East Coast Militia and Volunteer District. Between February 1870 and December 1871 his time was largely spent taking part in a series of expeditions against Te Kooti. These expeditions traversed extremely rugged terrain and involved difficult logistic and organisational problems. Porter acted as Rāpata's staff officer, dealing with matters such as pay and supplies, and as one of his principal subordinates. During the campaigns Porter 'was a marvel of energy and physical endurance', but his somewhat theatrical and egotistical personality later led him to consistently overstate his role.
In May 1877 Porter was given command of the East Coast Militia and Volunteer District. Increasingly, however, his time was taken up with his duties as native officer and land purchase officer. He was responsible for the implementation of native policy, and had to compile regular reports and carry out a wide range of administrative tasks. It is, however, Porter's land purchase work that is of particular importance. Although not officially appointed land purchase officer until 1875, from the early 1870s he procured large tracts of Māori land for the Crown; in the course of his career he purchased nearly one million acres. Porter's purchasing was facilitated by his command of Māori, his wife's family connections and his friendship with Rāpata. These relationships, however, also meant that Porter had a personal interest in some of the land deals he handled for the government, and led to possibly well-founded charges that he was using his official position for private gain.
In 1878, because he had been one of Donald McLean's men, Porter was removed from his official positions by George Grey's government. After John Hall became premier in 1879 Porter regained his land purchase and military posts. In the general election of 1881 he unsuccessfully stood for the seat of East Coast as a supporter of Hall. Porter ceased acting as a government land purchase officer in 1881, but resumed the position from 1886 to 1887, and again in the late 1890s.
Porter was promoted to major in 1885 and in 1889 he commanded the military force sent to Ōpōtiki to prevent Te Kooti from visiting Poverty Bay. He demonstrated considerable good sense and understanding in dealing with his old enemy. The following year Porter resigned as commander of the Poverty Bay district and was promoted to lieutenant colonel.
On 1 April 1900 Porter was appointed commander of the Wellington Militia and Volunteer District. A year later he was given command of the Seventh New Zealand Contingent in South Africa. Between May 1901 and May 1902 this contingent saw more action and suffered heavier casualties than any other New Zealand unit engaged in the war. It played a major and praiseworthy part in the fierce action at Bothasberg on the night of 23–24 February 1902. Porter also briefly commanded the Ninth Contingent, which served in South Africa between March and July 1902. For his service in South Africa he was appointed a CB and was mentioned in dispatches. In May 1902 he was promoted to colonel, and shortly afterwards took up command of the contingent which represented the New Zealand forces at the coronation of King Edward VII.
After Porter's return to New Zealand in October 1902, the minister of defence, Richard Seddon, with whom Porter was on friendly terms, gave him command of the Canterbury district against the strongly held views of the commandant of the New Zealand Defence Forces. Porter's period of command, from February 1903 to September 1904, was marked by controversy and allegations of political bias. Between October 1904 and June 1905 he was acting under-secretary for defence, the chief administrative post with the Defence Department in Wellington. During this time Herewaka Porter was in poor health; she died in Christchurch on 7 December 1904.
Porter took up the post of president of the Tai-Rāwhiti District Māori Land Council in June 1905. After the council became a board in November 1905, he held this position until mid 1908. At Petone on 6 January 1906 he married Florence Ellen Sheppard; they were to have no children. During his retirement Porter faced worsening financial problems, and in his last years his only income appears to have been a small annuity granted to him by Parliament in 1913.
During the First World War Porter was dominion commandant of the National Reserve. In December 1917 he was made inspector of recruiting services and given the task of assisting with the detection of men evading conscription. Porter's post was abolished in February 1919 and he died in Wellington on 12 November 1920, within hours of Parliament agreeing to increase his annuity. Florence Porter died in 1948.
In addition to his official duties Porter acted as a land agent, and had a variety of business and farming interests. By the early 1880s he had acquired substantial landholdings in rural Poverty Bay and in Gisborne. Porter built a large house in Gisborne, and played an active part in community affairs. He was an inaugural member of the Cook County Council and between 1878 and 1887 was mayor of Gisborne for a total of five years. He had a real interest in historical matters, assisting James Cowan with his research on the New Zealand wars, and writing a biography of Rāpata Wahawaha and biographical articles on Te Kooti.
Thomas Porter was a tall, powerfully built man with a distinctly military bearing. He was an ambitious and capable officer, with a particular affinity for Māori troops. He was also a rather conceited man with a marked taste for the trappings of success and social position. As a soldier, government official and settler Porter had a significant role in the development of European settlement on the East Coast.