William Hillier Onslow was born at Old Alresford, Hampshire, England, on 7 March 1853, the only son of George Augustus Cranley Onslow and his wife, Mary Harriet Anne Loftus. He was educated at Eton College and then briefly entered Exeter College, Oxford. In 1870, on the death of his great-uncle Arthur George, third earl of Onslow, he succeeded to the title and the family seat of Clandon Park in Surrey. On 3 February 1875 he married Florence Coulstoun Gardner, elder daughter of Lord Gardner, at St George's, Hanover Square, London. They were to have two sons and two daughters.
Onslow, following in a long family tradition of parliamentary service, had a distinguished career in the House of Lords. In February 1887 he was appointed parliamentary under-secretary of state for the colonies, and was vice president of the first Colonial Conference held in April that year. In February 1888 he was appointed parliamentary secretary to the Board of Trade.
At this time of economic stress (in Britain as in New Zealand) Onslow, whose rural estates were evidently hard hit, sought a salaried appointment as a colonial governor. Queensland was suggested, but on 24 November 1888 he was appointed governor of New Zealand. The New Zealand government had just cut the allowances for the governor's establishment and travel costs; this effectively downgraded the position by comparison with vice-regal posts elsewhere in the British Empire, and meant that it no longer attracted an experienced colonial administrator. Onslow was the first New Zealand governor since Robert FitzRoy in 1843 to have had no previous experience in vice-regal office, and, at 35, the youngest to be appointed since George Grey in 1845. Nor did he have the flair or flamboyance which helped some later governors win popular support.
Onslow assumed office on 2 May 1889 and with his wife took up residence in Government House in Wellington. A few months later there was an outbreak of typhoid in the town; their son and heir came down with the disease and for a time seemed in danger of his life. The Onslows were appalled and thereafter avoided Wellington whenever possible – a policy which did little for their popularity there.
However, Lady Onslow was at Government House on 13 November 1890 to give birth to their second son. As the first vice-regal child born in New Zealand, and in the colony's 50th jubilee year, it was variously suggested that he be given a distinctively New Zealand name, and that Queen Victoria might honour him and the colony as a godmother. After some negotiation both were arranged and the infant was named Victor Alexander Herbert Huia – the last after the native bird which symbolised nobility. Local sensitivities and the demands of protocol were satisfied by a christening ceremony in St Paul's Cathedral Church, Thorndon, Wellington, with the mayor, C. J. Johnston, as a godfather 'representing the people of New Zealand', and some months later by a hui at Otaki to present Huia Onslow (as he was always known) to Ngāti Huia who had in fact suggested his name.
Onslow, while noting the 'frightful jealousy' between the different towns and provinces of the colony, tended to discount signs of a developing national sentiment. For instance, in October 1889, as a prominent Freemason, he was approached by a group of local Masons seeking his patronage and support for a proposed Grand Lodge of New Zealand under which all the Masonic lodges of the colony, then under English, Scottish or Irish constitutions, could unite. Onslow hesitated, arguing that the time was not yet ripe – a position also taken by the leading Mason of Wellington (and premier) Sir Harry Atkinson. However, a majority of the Masons in New Zealand, including a large number of Atkinson's parliamentary colleagues (among them John Ballance, leader of the opposition) supported the new national body, which was successfully established in April 1890.
In 1890 Onslow told a friend, 'Colonials…are very easily led', but the results of his subsequent contests with ministers suggest otherwise. These contests arose over the constitutional issue of appointments to the Legislative Council which were made by the governor on the advice of his ministers. Up until 1890 Atkinson's government had refrained from advising any appointments to the Council. However, during that year his conservative followers, increasingly alarmed at the prospect of Ballance's Liberals gaining power, urged him to ensure a conservative majority in the Legislative Council. Onslow was by nature a conservative, and probably not unsympathetic to this viewpoint. In any case, he was familiar with the accepted practice in the British Parliament for appointments to the House of Lords to be made on the advice of even a defeated government. It was evidently with this precedent in mind that, when Atkinson raised the matter in May 1890, Onslow gave the rather unguarded assurance that a 'little list' of names for the Legislative Council could be agreed upon and held for appointing 'if things go wrong with you in the House or Country'.
At the election in December things did indeed go wrong for Atkinson's government, although until Parliament met again it was not clear whether Ballance's supporters would be numerous enough to take power. When rumour then spread of the 'little list' that would stiffen the Council against that eventuality, Onslow was quickly made aware that whatever the practice in Britain, it was widely felt in New Zealand that such an expedient on the part of a moribund government was neither usual nor acceptable.
Atkinson procrastinated, but finally forced the issue. Despite the public opposition to the appointments, Onslow could see no constitutional grounds either in his Royal Instructions or in colonial precedent for refusing to act on Atkinson's advice. He managed to have the number of appointments reduced from 11 to 6 (to a Council which then numbered only 39) but gave his assent, justifying his decision to the Colonial Office in terms of 'the constant practice in England'. In New Zealand, however, he was seen as aligning himself with the conservatives. Atkinson got his majority in the Council, but in the process its already shaky reputation was seriously undermined, and enough uncommitted members of the House were pushed into Ballance's camp to give him a clear majority when Parliament met a few days later.
Onslow found the new government members 'very earnest, very quiet, very conscientious'. Almost immediately another constitutional issue was settled without friction, when at Ballance's request and with the approval of the Colonial Office Onslow agreed to accept the advice of his ministers on the granting of pardons and reprieves – thus relinquishing one of the governor's remaining areas of responsibility.
In October 1891 Onslow sought leave to resign his position, pleading 'urgent and pressing' difficulties with his affairs in England. It appears also that he had found the costs of maintaining the dignity of his office heavier than expected, and, with the Wellington City Council still unable to act on the city's sanitation, he continued to fear for his family's health there.
Shortly before Onslow was due to sail, Ballance raised the issue of appointments to the Legislative Council again, seeking 18 appointments to counterbalance Atkinson's of the previous year and to fill recent vacancies. Onslow was unwilling to thus 'alter the complexion' of the Council by replacing a conservative majority with a liberal one; indeed, under his view of the constitutional function of an upper house it was axiomatic that the Council should always be conservative in approach, if not in political complexion. Onslow therefore indicated that he could agree to no more than eight appointments, but Ballance declined this compromise. Onslow was again placed in an awkward position. Rather than rejecting Ballance's advice and leaving his successor to deal with the consequences, he asked that the matter be deferred. This decision was hardly more popular than his earlier assent to Atkinson's appointments; in the event Onslow's successor, Lord Glasgow, proved so obdurate that the issue continued to bring the position of both governor and Council into question throughout his term as well.
One matter, however, was satisfactorily concluded before Onslow left in February 1892. He had become interested in the preservation of the native flora and fauna, and, building on the publicity surrounding the naming of his son Huia, had submitted a memorandum to the government (drafted for him by the prominent ornithologist Sir Walter Buller) urging the case for island sanctuaries for the disappearing native birds, and for statutory protection of his son's namesake, the huia, in particular. Ballance supported the idea and legal protection for the species was drawn up to be signed by Onslow on the eve of his departure.
Onslow returned to England and his parliamentary career in the House of Lords. He became parliamentary under-secretary of state for India in 1895, for the colonies in 1900, and reached cabinet rank in 1903 as president of the Board of Agriculture. He was chairman of committees in the House of Lords from 1905 until failing health forced his retirement in 1911. He died at Hendon, Middlesex, on 23 October the same year; his wife died in 1934.