William Swainson was born at Lancaster, England, probably on 25 April 1809, the eldest son of the merchant William Swainson. His mother's name is not known. He was educated at Lancaster Grammar School, admitted to the Middle Temple, London, on 6 June 1835 and called to the Bar on 8 June 1838. After practising as a conveyancer, Swainson was appointed by Lord John Russell in 1841 to be the second attorney general of New Zealand.
Swainson left Deal on the barque Tyne on 8 April 1841 with fellow passengers William Martin, the chief justice, and Thomas Outhwaite, registrar of the Supreme Court. They arrived, via Port Nicholson (Wellington), at Auckland on 25 September. The five month voyage was spent framing laws which would provide a basis for government in the colony. For Swainson, it was an opportunity to institute 'in simple, concise and intelligible language' a body of law unhampered 'by any complicated pre-existing system.'
The result was the passing of 19 enactments between 21 December 1841 and 15 March 1842. These provided for the establishment of courts of law, the creation of municipalities and the transfer of property. Only in the matter of land claims did Swainson find himself forced to withdraw and modify legislation which had sought to give legal effect to the Treaty of Waitangi. The Land Claims Bill introduced into the Legislative Council on 22 January 1842 was hotly contested by colonists before its withdrawal and replacement. A modified ordinance, passed on 25 February, was later disallowed by Lord Stanley, secretary of state for the colonies, in London.
The early clash of the colonists with the government over land claims was hardly unexpected. As a radical and a humanitarian Swainson felt the interests of the Maori to be most important. As a lawyer he was troubled by the vagueness of the treaty. He was concerned about the matter of asserting the Queen's sovereignty over tribes which had not signed, and about the extent of Crown authority: 'The natives were neither a conquered people, nor did a majority of them ever give an intelligent and unqualified submission to our rule'. Swainson held that only those Maori 'who had acknowledged the Queen's Authority could be considered British Subjects and amenable to British law.' Swainson took the treaty as his sole authority and tried to ignore British and New South Wales statutes which were in conflict with it. With Governor William Hobson's death in September 1842, Swainson lost a man on whom he relied and who trusted him.
The issue came to a head in late 1842 when feuding erupted between the Bay of Plenty tribes. Swainson and Martin resisted sending in troops against the chiefs Taraia and Tangaroa because they had not signed the Treaty of Waitangi. Instead, Swainson proposed the constitution of 'Native Districts' where the Maori could live under traditional custom. Willoughby Shortland, acting governor, and George Clarke, chief protector of aborigines, opposed this scheme, fearing that recognition of the 'Native Districts' would undermine British authority. The Colonial Office endorsed their stand and Swainson was strongly censured by Lord Stanley, who declared that 'the Queen has by the most solemn acts asserted Her own Sovereignty over the whole of New Zealand'.
When the Wairau affray occurred in 1843, Swainson was dispatched to assess the situation. He found that the claims of the colonists to the land had not been established, that the Maori were justified in the forcible repulse of the invasion, that the warrant for the arrest of Te Rauparaha was 'illegal and unjustifiable', that the European deaths were at the most manslaughter, and recommended that no action be taken. This attitude endeared Swainson neither to the settlers nor to Lord Stanley, who instructed Governor Robert FitzRoy to bring a militia bill before the Legislative Council. This was vigorously opposed by Swainson, Martin and William Brown, who argued that the Maori might become hostile if a militia were organised. Swainson urged that the bill be postponed for six months. In the absence of disciplined troops, and acutely aware that the anger felt by many settlers toward the Maori could lead to excessively harsh retribution, FitzRoy concurred. As a consequence, when Hone Heke rebelled and sacked Kororareka (Russell) in early 1845, FitzRoy and his administration were excoriated by the settlers for their perceived weakness. The Militia Bill was quickly passed and FitzRoy was recalled.
The arrival of George Grey in November 1845 saw a governor with sufficient troops to control an insurrection and a land policy to protect Maori interests. The Native Land Purchase Ordinance of 16 November 1846 restated the earlier principle (revoked by FitzRoy in 1844) of government control of the purchase of Maori land and sale to settlers. The objectives of Grey and Swainson in respect of land claims were parallel. Swainson and Martin assisted Grey in formulating proposals which were incorporated into the New Zealand Constitution Act 1852. Swainson was the first member appointed to the new Legislative Council and he became speaker on 16 May 1854.
It was a session of conflict. The settlers, led by Edward Gibbon Wakefield, sought to obtain responsible government. Colonel R. H. Wynyard rejected this as being outside his authority. This did not help the temper of the House. Wakefield tried to obtain the support of Swainson, without success. Wakefield resigned as adviser to the governor. Swainson's victory was shortlived. He left for England, on leave, and during his absence provision was made for him to be pensioned off at £400 per annum, two thirds of his salary. This took effect from 7 May 1856. Swainson remained a member of the Legislative Council until 1867, but did not attend after December 1864.
On his retirement there was little appreciation of his achievements. The land question was growing in bitterness, a situation Swainson had foreseen and tried to prevent. He never saw justification for the New Zealand wars and in December 1863 noted that the military themselves were becoming tired and disillusioned with the conflict. In his opinion the army was not engaged in the maintenance of sovereignty but in the acquisition of land, and this contest brought neither honour nor profit.
When he returned from taking leave in 1855–56 his expertise in law drafting was seized on by Bishop G. A. Selwyn, who was seeking to create an independent church which recognised its ties with the Church of England. Swainson was a member of the conference held in 1857 to prepare the constitution of the province. His skill in law drafting was seen in the framing of the fundamental measures introduced in the first General Synod in 1859. He remained a member for several years, and in 1866 became chancellor of the diocese of Auckland. He held this post until his death.
Swainson lived at Taurarua, a frame house he had brought with him in 1841. He purchased a property (over six acres) in Judges Bay, Auckland, for £98 14s. 9d. in 1844. This adjoined land owned by Eliza Hobson, which he subsequently purchased. In 1878 he sold part of his holding for £2,000.
William Swainson died at Taurarua from a 'cold…on his lungs' on 1 December 1884 and was buried at St Stephen's cemetery. Swainson was 'not a man known intimately by many and by no means an easy man to understand'. Nevertheless, at the time of his death, he was judged 'a good all round man' who was 'universally esteemed'. He was 'an enigma' to most people. In his will he requested that his papers be destroyed, but many of his ambitions and hopes were recorded by him in the books he wrote and in the commentaries of his contemporaries. In these he outlined his views of the manner in which the colony should develop and commented on the path it had already taken. He had been keen to see the establishment of responsible government which recognised the obligations entered into by the parties to the Treaty of Waitangi. His primary concern was that the Maori should be protected until they could exercise political power on an equal footing with the colonists. His guiding principle was that, 'We have covenanted with these people and assured to them the full privileges of subjects of the Crown.' He was aware of the responsibilities of the founding settlers and held that future inhabitants would 'receive the impress of its character from its present early founders.'