William Turakiuta Cooper (sometimes known as Wiremu Te Apatū Cooper) was born at Muriwai, Poverty Bay, probably in 1885 or 1886, one of twin sons of Robert Cooper and Mere Morera (or Christie). Both parents were of Ngāti Kahungunu and European descent. His mother was the daughter of Rawinia Te Apatū and grand-daughter of Te Apatū-o-te-rangi, an important chief of Ngāti Rakaipaaka of Ngāti Kahungunu. She was descended from the important Ngāti Kahungunu ancestors Te Huki and Tapuwae.
In 1908 William Cooper became a first-class licensed interpreter and worked in Nūhaka. He may also have worked for a time as a farmer and contractor. He and Kōparepare Matiu of Te Whakatōhea had a son, Matiu Te Auripo Te Hau, in 1912. In Gisborne on 24 July 1918 Cooper married Margaret (Maggie) Te Uira Paewai. By this stage he was living in Gisborne, where he was a leader of the Kahungunu Welfare Association. In October 1925 Apirana Ngata wrote to Prime Minister Gordon Coates saying Cooper 'was for very many years with Messrs Nolan & Skeet, Solicitors of Gisborne, and there showed a complete knowledge of the Native Land laws and practice, as well as of Maori history and custom'. In 1927 Cooper was native associate to the royal commission of inquiry into confiscated Māori lands that was chaired by Sir William Sim. In the same year he was involved in an inquiry into the sale of the Nūhaka lands of Ngāti Rakaipaaka in 1865.
After the 1928 election Ngata became native minister and inaugurated a programme of Māori land development. Cooper was placed in charge of land consolidation schemes in North Auckland: he was one of Ngata's specially selected men and women entrusted with this work and with encouraging a resurgence in Māori culture. In 1930 Cooper represented Ngāpuhi at a meeting at Waiomatatini and in 1932 he attended the Whakarewarewa land development conference at Rotorua.
Consolidation officers not only consolidated shares into economically viable blocks, but also arranged for surveys, bush and scrub clearance, fencing and roading. Cooper opposed the sale of land by Māori who had little other means of support. In this he agreed with Judge Frank Acheson who believed that earlier judges of the Native Land Court of the Tai Tokerau district had allowed excessive alienation of arable land, which had resulted in poverty for the sellers. In 1933 Cooper gave evidence to the Committee on Rating of Native Land and held that rates should reflect the land's productive value. He mentioned the difficulties in ensuring rates were paid on land in shared ownership and said that defaulting on rates by Māori should bring the land under the supervision of the Māori land development schemes. When he appeared before the committee he was described as a Māori welfare officer.
At the Rotorua land development conference in 1932 William Cooper had met Hohepine (Whina) Gilbert, née Te Wake. Whina became his adviser at Hokianga and she in turn valued his knowledge of farming, public service procedure and legislation. Later she began accompanying him on inspection tours of the Northland land development schemes. Her husband, Richard Gilbert, died in March 1935 and after his death William and Whina asked Maggie Cooper if she would agree to a separation, which she did: there were no children of the marriage and Whina was pregnant to William.
There was considerable opposition to the proposed marriage from the community at Panguru where Whina lived: she was a prominent Catholic and respected tribal leader and William was already married. The couple had to leave Panguru and went to live near Whāngārei. William continued to work for the Native Department and he and Whina later lived in Ōtīria. They had two sons and two daughters. Judge Acheson and Apirana Ngata visited them as before, and they were involved in electoral work for Tau Henare. They were active on the Tokerau District Rugby Football Committee and attended weekend race meetings.
In 1941 William and Maggie Cooper were divorced and the Catholic church found grounds to annul their marriage. William had previously been an Anglican and now became a Catholic. The couple were married in the meeting house at Ōtīria on 21 February 1941; they then lived at Te Karaka, where Whina had been born, and William travelled to and from his work in Whāngārei. In 1942 they returned to Panguru. William continued commuting and on one occasion was seriously injured when his car went off the road during a storm. In addition to his work, he managed Whina's farms and in the late 1940s the couple bought the 124-acre farm Tautoro from its Pakeha owner. William's twin brother, Eru, and his family came to live at Panguru on the Tautehere property, which Whina had bought in the 1920s.
The Coopers campaigned for a district high school for the area and Whina donated six acres of flat land at Panguru as the site for the school. After the government could not be persuaded to build more than a one-roomed school on the site, it was decided to move the local Catholic primary school there. At Panguru, when the school was being moved, William died suddenly on 4 August 1949. Whina recalled that earlier that night he had arranged his Māori and history books on shelves for his elder son to read.
William Cooper was of slight build and had a cast in one eye. He was held in high esteem for his work on the Māori land development schemes. At the time of his death he was a member of the Hokianga County Council. There was a large and representative gathering at his funeral and King Korokī later attended the unveiling of a memorial to him at Nāhareta cemetery.