William Henry Clayton was born on 17 November 1823, at Norfolk Plains, Van Diemen's Land (Tasmania). He was one of 12 children of Henry Clayton and his wife, Mary McLaughlan. William was educated at the local Longford Hall Academy, where he won prizes in Latin, French, mathematics and geography. Henry Clayton, a successful farmer and merchant, wanted his son to have the benefits of higher education, so the Clayton family sailed for England on 28 March 1840 on the Adelaide.
While in England William Clayton was articled to a prominent architect, and in the course of his architectural training he became proficient in surveying and civil engineering. On 7 October 1847, at Clapham, Surrey, he married Emily Samson. Soon after, the couple departed for Tasmania, arriving on 7 March 1848.
William and Emily Clayton spent the next 15 years in Tasmania. During this time their children – three daughters and three sons – were born. William achieved recognition as an architect: he designed over 300 buildings, including churches, mansions and commercial buildings, and was employed by the Survey Department for four years from late 1851 to late 1855. He also acquired status in community affairs: he was alderman in Launceston from 1857 to 1863 and in 1858 was appointed a justice of the peace.
Attracted by business opportunities arising from the success of the Otago goldfields, William Clayton emigrated to New Zealand, arriving at Dunedin on the Omeo on 29 April 1863. His family followed soon after. By 1864 he had allied himself with William Mason, a well-established New Zealand architect; they practised under the name of Mason and Clayton. During his six years in Dunedin Clayton designed many buildings, the most notable being the Colonial Museum, Wellington (1864), All Saints' Church, Dunedin (1865), Edinburgh House, Dunedin (1865), Otago Provincial Government Buildings (1867), and the Bank of New South Wales building in Christchurch (1867). He also appears to have made a major contribution to the design of the Exhibition Building, Dunedin (1865), for which the partnership of Mason and Clayton was awarded the silver medal. Clayton was invited to act as a juror for 'Civil Engineering, Architectural and Building Contrivances' at the exhibition.
In Dunedin wealth allowed architecture to flourish, but by 1869 the decline in gold returns and wars during the past 10 years had impoverished the whole colony. On 5 April 1869 Clayton wrote to the colonial secretary, E. W. Stafford, offering his services as colonial architect; within days his offer was accepted. His salary was set at £200 per annum with provision for a 2½ per cent commission on contracts exceeding £200. He was also allowed to continue his private practice. The Claytons left Dunedin for Wellington on 12 May 1869 on board the Airedale.
William Clayton's appointment as New Zealand's first (and only) colonial architect and his move to Wellington coincided with the commencement of Julius Vogel's schemes for renewed immigration and large-scale public works. Clayton was charged with providing the public facilities made necessary by such expansion. Customs houses and immigration and quarantine barracks were required at major ports. The social implications of accelerated development meant that more court houses, prisons and lock-ups were needed. The construction of schools, offices, government employees' cottages and hospitals all became Clayton's responsibility. Clayton devised standard plans to answer the problem of quickly erecting numerous small wooden buildings throughout the colony, from the goldfields in the southern provinces to the north of Auckland. At least 80 post offices alone were constructed between 1870 and 1877. The standard-plan buildings were successful because of their appropriate simplicity, but their appearance belied the wealth of engineering and architectural experience which generated them. At first Clayton was attached to the colonial secretary's office, but in 1873 he was transferred to the Public Works Department as head of the architectural division.
Clayton also designed substantial public buildings, of which the government departmental offices in Wellington, known as the Government Buildings, was the largest and most important. Completed in 1876, it is unusual in that it appears to be a masonry building, but is constructed in timber. Closest in spirit and scale to Renaissance architecture, this building is a memorial to William Clayton as architect. It is still the largest wooden building in the southern hemisphere. Other notable public buildings which Clayton designed while colonial architect were Government House, Wellington (completed 1871), the new Legislative Council Chamber (old Parliament Buildings), Wellington (1873), Custom House, Russell (1870), Telegraph Office, Dunedin (1876), and the general government buildings in Invercargill (1876), Lyttelton (1875) and Christchurch (1879). He also designed a residence for himself in Hobson Street, Wellington. This house, completed in 1874, was the first concrete house in New Zealand and the first in Wellington to have hot and cold running water. It later became part of Queen Margaret College.
In 1867, at Dunedin, William Clayton's daughter, Mary, affectionately known as Polly, had married Julius Vogel, who was at that time a member of the House of Representatives. In the following years William saw little of Mary, although they corresponded regularly. In the second half of 1877 Emily and the children left to visit the Vogels who were at the time residing in England. During their absence William journeyed south to Christchurch and Dunedin to value the old provincial councils' buildings.
He was in low spirits after parting from his family and on arrival in Dunedin suffered pain from an old ankle injury. The limb became so infected that his medical attendant, Dr T. M. Hocken, strongly advised amputation. Seven days after the operation, on 23 August 1877, Clayton died. He was buried in Dunedin's Northern cemetery the following day. He was only 53 and an obituary suggested that 'but for the effects of the accident [he] might in all human probability have lived many years longer, as he was a man with a naturally strong constitution'. Emily Clayton returned to New Zealand to settle her husband's estate and then returned to London where she spent the rest of her life. The headstone on Clayton's grave was erected by his daughter, Mary Vogel.