William Spain was born at Cowes, Isle of Wight, on 14 March 1803, the fourth child of George Spain, merchant, and his wife, Sarah Rosendal. William was educated at Hyde Abbey School, Hampshire, trained as an attorney at Winchester and qualified in 1823. On 9 April 1825 at Watton-at-Stone, Hertfordshire, he married Mary Elizabeth White. Mary and William Spain had eleven children, two of whom died in infancy. A supporter of the Whig party, Spain became a central secretary for Hampshire, was active during the passage of the 1832 Reform Act, was involved for two years with the work of a New Zealand committee, and acted for some time as a private secretary to Lord Palmerston.
In January 1841 the secretary of state for the colonies, Lord John Russell, appointed Spain to the position of land claims commissioner in New Zealand. He was to investigate the claims of the New Zealand Company which held that it had bought some 20 million acres of land around Cook Strait in 1839. The company had already sent hundreds of settlers there by the end of 1840. On 20 April 1841 Spain and his family sailed from Gravesend for New Zealand on board the emigrant ship Prince Rupert. Among their fellow passengers were the colony's new surveyor general, Charles Ligar, and five assistant surveyors. In early September the Prince Rupert was wrecked without loss of life near Robben Island, Table Bay, south-west Africa. The commissioner's party and the surveyors continued their journey in a chartered brig, Antilla. They reached Auckland on 24 December 1841.
Although Russell had envisaged Spain's dealing with non-New Zealand Company claims, Governor William Hobson felt it was too big a job for one man to deal with in a reasonable time. Hobson already had two land commissioners at work on the non-New Zealand Company claims. These were Captain Mathew Richmond and Colonel Edward Godfrey, who had been appointed in September 1840 by Governor George Gipps. Hobson therefore decided to retain all three commissioners. Spain's work was to be confined to investigating the New Zealand Company claims and any non-company counter-claims to the same lands. He was instructed to give effect to the current Land Commission Act and also to two agreements made between the company, and the British and New Zealand governments in November 1840 and September 1841 respectively.
The commissioner began work in Wellington on 15 May 1842. The company's claim to the Wellington district rapidly collapsed in the Land Claims Court and this was paralleled by increasing Maori opposition to European occupation of town and Hutt Valley lands. This encouraged the New Zealand Company agent, Colonel William Wakefield, to offer to compensate those Maori who had missed out on the general payments made in 1839. Both Spain and the acting governor, Willoughby Shortland, considered this suggestion to be the most practical solution. Under the scheme, compensation was to be paid at the same time as the claims were investigated. This changed the focus of the commission's work from deciding which land was company land and which was Maori, to deciding where compensation was due and whether the original purchase was valid. The only lands to be excluded from alienation in this way were pa, cultivations, and burial grounds. In spite of this favourable development, Wakefield did all he could to obstruct Spain's investigations and it was not until the Wairau incident in June 1843 that he began to co-operate. The compensation scheme, and Wakefield's delaying tactics, made the Maori claimants increasingly sceptical of the land court's impartiality and deprived the commissioner of the earlier influence he had enjoyed with the Maori.
During his investigations Spain decided that the company had made valid purchases in only two of the areas it claimed: Manawatu and New Plymouth. Nevertheless, he awarded all the land it claimed to the company and compensation to the Maori. The only lands he did not award, because of Maori resistance, were the Manawatu and Porirua districts. However, in every instance, the compensation was accepted reluctantly or refused. Governor Robert FitzRoy issued only two Crown grants on the basis of Spain's awards because of Maori opposition to occupation of the other lands and their demands for further substantial payments. The Crown grants, signed in late July 1845, were for Wellington and Nelson. Final settlement of the New Zealand Company's other claims did not occur until years later and was achieved in several ways, notably through land purchase and military force. In spite of the dissatisfaction with Spain's awards at the time, his thorough groundwork and the substantial files compiled during his investigations provided invaluable information for later settlements of the claims to lands in the Cook Strait region.
Spain and his family moved to New South Wales in 1845, where he practised as a solicitor until 1851. He was then appointed as the first inspector general of police and became a member of the Legislative Council of New South Wales. He resigned from both offices in December 1851. Spain returned to private practice until 1856 when he was reappointed to the Legislative Council. He held this office until 1858. In 1861 Spain retired to Waverley, Sydney. He died at nearby Woollahra on 5 April 1876.