According to family information William Williams was born at Plumtre House, Nottingham, England, on 18 July 1800, the ninth and youngest child of Mary Marsh and her husband, Thomas Williams. He was baptised on 30 October 1800. Thomas Williams was of Welsh descent, a hosier by trade and a man of substance in Nottingham. He was a Dissenter, but never accepted the Unitarian doctrine so strongly propounded in Nottingham's chapels during the eighteenth and early nineteenth centuries. He died of typhoid when William was three. After an unsuccessful attempt to carry on the hosiery business Mary Williams moved with her younger children to Southwell, Nottinghamshire, where she began a school for young ladies.
In 1813 the marriage of William's sister, Lydia, to their cousin Edward Garrard Marsh brought the family under the influence of this evangelical clergyman. Marsh interested Henry, one of William's older brothers, in the work of the Church Missionary Society, which in turn affected William. Another consequence was that members of the Williams family turned from nonconformity to the Church of England. This dissenting, evangelical background considerably influenced the two missionary brothers and was shared by their wives, making them opponents of all later high church practices within the Anglican church.
William Williams was educated at a small dame school and at Southwell Grammar School. He completed an apprenticeship to a Southwell surgeon before entering Magdalen Hall (later Hertford College), Oxford, in 1822, as a prospective CMS trainee, under the special care of its evangelical principal, Dr John Macbride. He came down from Oxford in 1824 with a BA in Classics, and the same year was ordained deacon, on 26 September, and priest, on 19 December. At the beginning of 1825 he was at the CMS Training College, Islington, London.
From the outset of his missionary training there had been a tacit agreement with the CMS that he should follow his brother, Henry, to New Zealand. During a fund raising tour of the Midlands news of his imminent departure reached William and hurried along marriage plans. At Sheffield, on 11 July 1825, he married Jane Nelson of Newark, Nottinghamshire, and on 12 August they embarked on the Sir George Osborne. After a three month stay at Sydney they landed at Paihia, Bay of Islands, on 25 March 1826. Between 1826 and 1846 they had nine children, all born in New Zealand.
At Paihia William Williams was in charge of the English boys' school and, until the arrival of Samuel Ford in 1837, was the mission doctor. His early fluency in spoken Māori was noted by Henry Williams: 'He…appears not to learn it; but it seems to flow naturally from him'. In September 1826 he began the first serious, sustained effort to produce the Scriptures in Māori. By the end of 1837 he had completed the whole of the New Testament and the greater part of the Book of Common Prayer.
In May 1835 the English boys' school was relocated at Waimate North, which became William's second station. He had already made several missionary journeys, some of them most important. In December 1833 and January 1834 he had gone by schooner to the East Cape and Māhia peninsula, accompanied by William Yate, to return Ngāti Porou Māori captured by raiding Ngāpuhi. (These people were to become the forerunners of the CMS East Coast mission.) Between July and November 1834 he had travelled overland to the Thames and Waikato regions, accompanied by Alfred Nesbit Brown. In January 1838, with William Colenso, Richard Matthews and James Stack, he made an overland journey from East Cape to Tūranga, Poverty Bay. He was determined that a CMS missionary be stationed on the East Coast, and when Richard Taylor, who had travelled with him on another visit there from March to May 1839, agreed to take over the Waimate school, he and Jane left for Tūranga on 31 December 1839.
Apart from a visit to England during 1851–52 to vindicate the New Zealand mission and his brother, William Williams remained based at the Tūranga mission station from 20 January 1840 to 3 April 1865. For many years he was the only ordained CMS missionary in the church's eastern district, walking north to East Cape, south to Hawke's Bay and inland to Waikaremoana as part of a regular visiting schedule. He made occasional overland journeys to Wellington and to St John's College, Auckland. Selwyn inducted him as archdeacon of the East Cape on 27 November 1842, and on 3 April 1859 consecrated him bishop of Waiapu, a diocese which initially had a predominantly Māori character. (On his English visit a doctorate of canon law from Oxford had been conferred on him.)
In April 1857, having come to realise that the training of a Māori pastorate was his main job, William Williams moved from the first mission site at Manutuke (at Kaupapa between 1840 and 1844, and then at Whakatō), to locate his Māori training schools and his residence at Waerenga-a-hika, a few miles inland, where there was more land available for a mission farm. After leaving Tūranga in 1865 he stayed for two years at Paihia where he began another training school at Horotutu. There he wrote Christianity among the New Zealanders, published in London in 1867 and intended as an apologia for the CMS mission in New Zealand. At the end of May 1867 he moved to Napier and the following year into his final residence, Hukarere, on Napier hill. An agreement between Bishops G. A. Selwyn and C. J. Abraham had added Hawke's Bay to the Waiapu diocese, and William was anxious to make Te Aute estate (set aside for educational purposes by his nephew and son-in-law, Samuel Williams) the site of his central diocesan school. In July 1875 he also established the Hukarere school for Māori girls, close by his own home. His daughter, Anna Maria, was principal. On 9 February 1878 he died at Hukarere. His land at Napier was worth nearly £9,000, and he left other property at Kerikeri, Tauranga and Gisborne.
William Williams once described his missionary life as 'like the unbroken course of a parish schoolmaster. A great deal of work, but most of it of the same character'. With his Māori converts he regularly 'read and conversed', but apart from his knowledge of the language he showed little interest in Māori culture and disapproved of most Māori social customs. Nevertheless his influence among his mission Māori, to whom he was known as Parata (Brother), was considerable. He generally found that 'a little quiet expostulation' settled differences between Māori and missionary. His colleagues found him kindly, easy to get along with and 'a gentleman', but when his principles were crossed, either by Bishop Selwyn or by the CMS secretaries in London, he was adamant and resolute. His decision to quit Waerenga-a-hika in 1865, when it was threatened by a small band of Hauhau who fraternised with his Tūranga Māori, appears to have been influenced not so much by the admonishments of Selwyn and members of his family, as by William's own determination to withdraw his presence and his mana from those who were prepared to entertain 'false gods'.
His attitude to colonisation and to the New Zealand wars changed as he grew older. In 1840 he collected signatures to the Treaty of Waitangi, and later defended its land guarantee against threats by settlers and British authorities. He was critical of the Waitara purchase, but thought that the wisest course was for the government to subjugate 'rebel' Māori; 'salutary chastisement' would bring them to their senses. Later he revised that opinion: 'All this war down to the present time  has sprung out of Waitara…. As a community and as a government we have been puffed up, first with an idea that we were in the right, & secondly that we were able to put down the natives by our own strength…. We are now brought very low.' Land confiscation, he came to think, was particularly unjust. For years he had regarded Tūranga as a missionary enclave; returning there from England in 1853 he disapproved of the attempt made by his locum, T. S. Grace, to introduce European trading practices.
As a steady, conscientious teacher William Williams was neither too uplifted by the apparent missionary success of the 1830s and 1840s, nor too dismayed by the massive falling away of the 1850s and 1860s. All through his missionary life he kept revising the Māori New Testament and Book of Common Prayer. In 1844 he was with the 'Translation Syndicate' at Waimate, but mostly he worked alone, conferring from time to time with Robert Maunsell. His enduring memorial is A dictionary of the New Zealand language, first published at Paihia in 1844. The second edition was also his work, the third and fourth that of his son, Bishop William Leonard Williams, and the fifth, of his grandson, Bishop Herbert William Williams.