Walter Baldock Durrant Mantell was born at Lewes in Sussex, England, on 11 March 1820, the second child of Mary Ann Woodhouse and her husband, Gideon Algernon Mantell. Gideon Mantell, a medical practitioner, was also a prominent palaeontologist and geologist, who was in touch with the leading savants of his day. He hoped to steer Walter into his profession, sending him to schools at Brighton and Tottenham, then to medical school at London University, and apprenticing him to a surgeon at Chichester. But Walter did not complete his training and on 18 September 1839, though not yet 20, suddenly departed on the Oriental for the New Zealand Company settlement at Wellington, where he arrived on 31 January 1840.
Like many of the romantic young gentlemen who were beguiled by the propaganda of the New Zealand Company, Mantell came out with no fixed career in mind. But he had the skills and contacts to find preferment. He tried his hand at farming, but on 23 December 1840 was appointed clerk to the Bench of magistrates, and deputy postmaster at Wellington. He had given up the former position by October 1841, and in 1842 the latter position was combined with that of clerk to the acting sub-collector of customs. Mantell was twice threatened with redundancy before his resignation in February 1844. His positions were far from onerous, and he found time to assist with the formation of the company settlements at Whanganui and New Plymouth, and to indulge his passion for natural history. Then from 1845 to 1848 Mantell was a superintendent of military roads at Porirua, a job that at least allowed him to learn Māori from his workforce, thus preparing the way for his next and far more important appointment.
In August 1848 Mantell was appointed to the office of commissioner for extinguishing native titles, Middle Island (South Island), with the initial responsibility of setting aside reserves for Ngāi Tahu within the Canterbury block, recently purchased by Henry Tacy Kemp. Mantell was instructed to induce Ngāi Tahu to combine their numerous settlements into as few localities as possible, while making 'liberal provision … for their present and future wants'. However, this 'liberal provision', according to Governor George Grey's instructions, involved limiting the reserves to 10 acres per head of Ngāi Tahu population. Mantell did just this, reserving 6,359 acres for an estimated population of 637, out of a block estimated to contain 60 million acres – all this in the face of what he saw as the sullenness and 'determined resentment' of Ngāi Tahu. Their resentment was not surprising, since Grey had earlier promised them that if they sold the block they would receive reserves 'for their present and reasonable future wants' and the government would provide them with schools, hospitals and general care.
At this time Mantell was content to serve the interests of government, to which he looked for future employment; his services were duly rewarded. In 1849 he completed the purchase of two blocks, and commenced the purchase of a third, on Banks Peninsula, in the face of considerable opposition from Ngāi Tahu, who, as he put it, 'conducted themselves, as usual, in the most insolent and turbulent manner'. No subsequent payment was made and the whole of the peninsula, apart from land set aside in two reserves for Ngāi Tahu and for the French Nanto-Bordelaise company, was regarded as Crown land.
In October 1851, as a result of the 'able and satisfactory manner' in which he had carried out his earlier purchases, Mantell was entrusted with the purchase of the Murihiku block. This huge area, comprising the whole of the south-west portion of the South Island, from the Nuggets on the east coast to Milford Sound on the west, was purchased on 17 August 1853 for £2,600. This time Mantell provided somewhat larger, although far from princely, reserves – altogether seven were set aside, with an average size of 33 acres per person. In June 1853 Grey approved Mantell's establishment of two small foreshore reserves at Dunedin and Port Chalmers where Ngāi Tahu could build houses and beach their boats, but which were soon surrounded by the spreading townships.
At this time Mantell came into conflict with William Cargill, who had been appointed commissioner of Crown lands within the Otago block in January 1851. Mantell himself, in October 1851, became commissioner of Crown lands for the Southern District of the Province of New Munster, with responsibility for the settlement of Europeans on the lands purchased from the Māori. Indeed, some squatters had already occupied pastures in the Murihiku block while it was still in Māori ownership, thus lending urgency to Mantell's quest to complete the purchase for the Crown. When Cargill became superintendent of Otago in September 1853 and demanded control of Crown land, a stalemate was reached, and in December 1853 Mantell was instructed to assume the administration of the Otago block. The dispute was exacerbated by Mantell's penchant for ridiculing the puritanical Scots of Dunedin and suspicions that Cargill was trafficking in land orders. Mantell took leave of absence and returned to England in 1855.
Mantell was becoming preoccupied by a concern that was to haunt his conscience and affect his career for the rest of his life: the non-fulfilment of promises he and others had made to Ngāi Tahu at the time of the original land purchases. Just before his departure Mantell complained that, on reminding the government of the unfulfilled promises, he was referred to the General Assembly, in which neither the imperial government nor the Māori were represented. Unsatisfied in New Zealand, Mantell appealed to the secretary of state for the colonies; he was refused an interview and resigned his New Zealand appointment. He returned to New Zealand and took his case to the General Assembly, being elected to the House of Representatives for Wallace in 1861.
Mantell's chequered parliamentary career was affected by his temperamental personality and persistent but unavailing attempts to rectify the broken promises to Ngāi Tahu. In July 1861 he accepted office as native minister in the Fox government, on condition that the promises to Ngāi Tahu would be fulfilled. He resigned six months later when that condition was repudiated. He again accepted office in the Domett and Weld ministries, on the same conditions, and resigned from both when the promises were not fulfilled. In 1866 he retired from the House but accepted a seat in the Legislative Council, which he retained until his death. In various government inquiries into Ngāi Tahu claims, such as the 1888 Joint Committee on the Middle Island Native Claims, Mantell remained a persistent advocate of the Ngāi Tahu cause. He resented the procrastination over settlement of the claim and felt that he had been unwittingly led to negotiate under false pretences.
If Mantell's involvement in Māori affairs and politics was a cause of much anguish, his pioneering work in natural history brought him considerable satisfaction and some fame. Through his father's connections Walter Mantell was in regular contact with scientists such as Charles Lyell, the doyen of British geology, who asked Mantell for information on earthquakes, and Charles Darwin, who asked about glacial action, a possibly extinct hairy reptile and the Māori conception of beauty. He answered their inquiries as best he could.
Mantell's most notable association was with Richard Owen, superintendent of the natural history department of the British Museum. Mantell supplied him with collections of specimens, including numerous crates of moa bones, which he dug from the Waingongoro site in South Taranaki in 1847 and collected from various South Island sites during his land-buying expeditions in the early 1850s. He hoped that payments for the collections would cover his costs; his father, seeking to promote Walter's discoveries to learned societies in London, hoped also that they would facilitate his career in the public service in New Zealand. Gideon Mantell's death in 1852, before the arrival of Walter's large South Island collection of moa bones, removed an important link in the scientific chain. But Walter Mantell's work with Owen at the British Museum during his sojourn in London in 1856 resulted in the reconstruction of the largest moa skeleton yet recovered, Dinornis elephantopus. In Britain it was Owen who gained fame and fortune from the moa discoveries, although as Mantell noted sourly some years later: 'He has made considerable blunderings & flounderings in his search after renown rather than truth.'
Mantell's position was typical of the colonial relationship that then existed in science: Mantell, the young collector, was providing grist to the intellectual mills of the savants in London. But in New Zealand, at least, Mantell had the respect of his fellow scientific workers. He was in frequent contact with other leaders in the moa hunt, including William Colenso, Julius von Haast and James Hector. He discovered and gave his name to Notornis mantelli, and he was active in the affairs of several learned societies, including the Wellington Philosophical Society and the New Zealand Institute. From time to time, during Hector's absences, Mantell was acting director of the Geological Survey and Colonial Museum. And he was sometimes used by government to act as a commissioner; for instance, for the Philadelphia Centenary Exhibition in 1876 and the Australian Exhibition in 1879. He was a handy man, who, however, never won the top prizes in science, administration or politics.
Mantell married Mary Sarah Prince at Wellington on 5 August 1869; their only child, Walter Godfrey, born in 1864, was legitimised in 1894. Mary Mantell died in 1873, and on 10 January 1876, at Wellington, Mantell married Jane Hardwick; they had no children. Mantell died at Wellington on 7 September 1895.