Frank Oswald Victor Acheson was born on 27 June 1887 at Riverton, Southland, the youngest of nine children of Robert Acheson, a merchant, and his wife, Annie Sinclair Allan. He was educated at Riverton school and Southland Boys' High School (1901–2). In 1903 he began work as a clerical cadet for the civil service in Wellington. He later went to Whanganui and then to Dunedin, where in 1907 he started studying for an LLB at the University of Otago.
On 1 June 1910 at Whanganui Frank Acheson married Flora Catherine McGregor. In 1912 they moved to Wellington, where their only child was born. Frank worked for the civil service while studying at Victoria University College, in 1913 obtaining his LLM.
In 1914 he moved to the Native Department to work as a clerk to the Native Land Purchase Board. The depth and originality of Acheson's interest in Māori land tenure is demonstrated by a long essay he prepared in 1913 for the Jacob Joseph Scholarship at Victoria University College. Here Acheson argued against the prevailing belief that Māori lacked a body of customary law governing land tenures. Instead, he insisted that there was a clear and comprehensive body of custom and practice; by analogy with international law he also attacked the view that Māori customs were not truly legal because they lacked a sovereign authority to enforce them. He analysed in detail Māori law relating to wills, adoption and the legal position of vassals. His work was marked both by a detailed knowledge of Native Land Court practice and by a sophisticated understanding of jurisprudence and international law. In his essay Acheson consciously set himself against the dominant legal opinion of the day.
In 1918 he was promoted to the position of native land purchase officer. He spent much time in Hawke's Bay and Whanganui, developing a good knowledge of Māori society and of the principal sellers and non-sellers in the Māori-owned blocks in the lower half of the North Island. Sometimes he intervened in a direct and paternalistic way in the sale process by ensuring that the proceeds for particular individuals were paid into trust accounts. Acheson's brilliance and expertise led to his appointment in 1919 at the age of 32 as commissioner and a judge of the Native Land Court. From 1919 to 1924 he was judge of the Aotea (Taranaki–Whanganui) district of the court. During these years he became friendly with the Te Heuheu family, the leading family of Ngāti Tūwharetoa.
In 1924 Acheson became judge of the Tokerau (North Auckland) district, beginning a long period of close involvement with the Māori tribes of Northland. He also became the president of the Tokerau District Māori Land Board. Acheson was aware of the shocking poverty of the Māori communities of Northland and was determined to ameliorate their poor living conditions. He believed that this could only come about by establishing long-term development schemes through which Māori would be encouraged to develop their remaining tribal lands.
In 1925 Frank Acheson proposed setting up a dairying scheme at Te Kao with start-up capital to be provided from the accumulated board funds. The scheme involved title consolidation, land development, and the establishment of a cream truck service between Te Kao and Kaitāia. By 1930 a considerable portion of the board's funds were tied up in the Te Kao scheme. The 1934 commission of inquiry into the administration of the Native Department (although supportive of Acheson's work in the far north) and the election of the Labour government in 1935 led to increased official supervision of the land development schemes. Acheson's impatience with bureaucratic formalities led to a bitter feud with his registrar, J. H. Robertson. Their relationship became impossibly strained and this seriously affected Acheson's dealings with the Native Department and with the Labour native ministers, Frank Langstone and H. G. R. Mason. Acheson regarded Native Department officials as insensitive and unimaginative; for their part staff regarded Acheson as careless of financial details and too partisan towards Māori. In 1938 Acheson levelled charges of disloyalty and obstruction against Robertson, which led to an inquiry by the public service commissioner. The commissioner, however, vindicated Robertson, who remained as registrar in Tai Tokerau until after Acheson's retirement. He and Acheson seldom spoke to each other.
Acheson's judicial decisions were often innovative, and sometimes caused alarm in official circles. He was involved in a series of cases relating to Māori claims to the foreshore, which were strongly opposed by the Crown, represented by Vincent Meredith, Crown solicitor at Auckland. The point at stake was whether the Native Land Court could issue titles to the foreshore. Acheson contended that it could; his decisions were appealed by Meredith to the Native Appellate Court, which did its best to devise compromises. In a case concerning Lake Tāngonge, near Kaitāia, Acheson defied legal orthodoxy by insisting that he could take 'judicial notice' of the Treaty of Waitangi. In his 1929 judgement on Lake Omapere, near Kaikohe, he ruled that Māori customary law recognised the ownership of lakes, that at 1840 the lake was the property of Ngāpuhi, and that Ngāpuhi's rights were protected by the Treaty of Waitangi.
Acheson also played an important part in the long-running Ōrākei case at Auckland. Following petitions from Ōrākei Māori, he conducted an inquiry and prepared a report on Ōrākei in which he concluded that the Ōrākei block should have been protected from alienation. He also proposed that representatives of Ngāti Whātua and the Crown should meet to resolve the matter with no officials present. Acheson's recommendations, however, were effectively overruled by Chief Judge R. N. Jones when forwarding Acheson's report to Parliament in 1932 and the government took no action.
As a judge Acheson became friendly with many prominent Māori families, and it was largely at his urging that Ngāti Tūwharetoa decided to provide most of the cast and locations for a feature movie, Hei, tiki, produced by an American, Alexander Markey. Acheson was an adviser and supporter of Te Puea Hērangi in Waikato, and worked closely with Northland Māori leaders, such as Whina Cooper, actively backing their project to build a canoe for the 1940 centennial of the Treaty of Waitangi.
Frank Acheson published various papers and articles, including six in the Mirror in 1940 on Te Puea's work. In 1930 he published Plume of the Arawas, a romantic adventure novel which idealises traditional Māori life. It sold well and went into a second edition in 1938. Acheson was a passionate rugby enthusiast. He was a capable player until injured, and in 1924–25 was part of a small group of official supporters of the All Black tour of Britain, France and Canada.
By 1940 the Te Kao dairy scheme had resulted in a complicated dispute between Frank Acheson and the Native Department. Acheson's stance was that once the board's advances had been repaid, the assets and profits belonged to Te Kao Māori; the department's view, backed up by a Crown law opinion, was that the assets belonged to the board and that the profits had to be paid into the board's general fund. Matters became fraught and acrimonious.
Acheson disliked the Labour government, and in 1943 he narrowly missed selection as National candidate for Rotorua. Subsequently Mason advised Acheson that he was to be compulsorily retired at the end of 1943. The ostensible reasons were the alleged insolvency and illegal expenditures of the Tokerau Board, but it is likely that Acheson's relations with the government, his strongly pro-Māori stance, and his constant criticisms of the conduct of Crown officials influenced the decision to dismiss him.
Acheson remained in Northland for a time, helping with the campaign of Te Aupōuri people to retain control of their assets, but when community leaders began to negotiate with the government he admitted defeat and quietly left the north, never to return. His wife, Flora, had died in 1943 and his daughter was living in the United States. He returned to Southland, becoming chairman of the Southland Regional Planning Council. He tried once again to obtain selection as a National Party candidate, this time for Wallace in 1946, but was unsuccessful. In 1947 he was elected mayor of Riverton. In December that year he suffered the first of three strokes and died on 25 March 1948 at Invercargill, survived by his daughter.
Frank Acheson earned considerable affection and respect in the Māori world and was adopted as an honorary rangatira by Ngāti Tūwharetoa, Te Arawa, Waikato and Ngāpuhi. He was an innovative Native Land Court judge at an important stage of the court's history. Regarded as partisan and unorthodox by his contemporary opponents, he now seems to have been a man ahead of his time. Only with the resurgence of historic Māori land claims in the Waitangi Tribunal has Acheson's work begun to receive long-overdue recognition.