Hanan was born in 1909 in Invercargill.^[1] He was the son of the draper James Albert Hanan and his wife, Johanna Mary McGill. His uncle and aunt were Josiah and Susanna Hanan.^[2] He received his education from Southland Boys' High School, Waitaki Boys' High School, and the University of Otago, from where he obtained an LLB.^[1] He returned to Invercargill and practised law from 1935.^[1] In 1939, he went into partnership with Ian Arthur, practising as Hanan Arthur and Company. In 1940, he enlisted for war service.^[2]

On 3 March 1939, he married Ruby Eirene Anderson, known as Eirene, at Invercargill's St Paul's Presbyterian Church.^[2]

Hanan was elected to Invercargill City Council in 1935. Three years later, he was elected Mayor of Invercargill. He relinquished the position in 1941 so that he could participate in the war.^[2][3] His uncle had previously been Mayor of Invercargill (1896–1897).^[4]

He served with the 20th Canterbury-Otago Battalion in the Middle East and in Italy. He was wounded at the outbreak of Minquar Qaim. He would have died had it not been for a truck driver who found him unconscious, put him onto the back of the lorry and took him away.^[5] The injuries resulted in a serious lung condition that saw him sent home in 1944 as an invalid.^[1] He had attained the rank of captain during the war.^[2]

He represented the Invercargill electorate in Parliament from 1946 to 1969, as had his uncle before him (1899–1925).^[6] He held positions as Minister of Health (1954–1957), Minister of Immigration (1954–1957),^[7] Attorney-General (1960–1969), Minister of Justice (1960–1969), Minister of Māori Affairs (1960–1969), and Minister of Island Territories (1963–1969).^[8][9]

In 1961, Hanan and nine other National MPs (Ernest Aderman, Gordon Grieve, Duncan MacIntyre, Robert Muldoon, Lorrie Pickering, Logan Sloane, Brian Talboys, Esme Tombleson and Bert Walker) crossed the floor and voted with Labour to abolish the death penalty for murder in New Zealand. As Minister of Justice, it was his responsibility to introduce the law to Parliament, but he did so by saying that he disagreed with it.^[2] He convinced enough of his party colleagues to vote with the opposition and thus abolished the death penalty in New Zealand, which is what he is best remembered for.^[5]

In much of his political work, Hanan was able to read the mood of the public well and he was guided by this. On many occasions, he developed policy that was initially not accepted by his party colleagues, but he managed to talk them round to it.^[2] One controversial piece of legislation that he introduced was the Maori Affairs Amendment Act 1967, which was bitterly opposed by many Māori, as they feared that it would lead to further loss of land.^[10] Hanan was also an early supporter of homosexual law reform, writing to the New Zealand Homosexual Law Reform Society that he commended their goal of "replacing prejudice and emotion by understanding and a rational approach."^[11]

Hanan belonged to the powerful inner circle of the Holyoake cabinet. When two of the inner circle, Hanan and Tom Shand (Minister of Labour), died within months of one another, Holyoake's strong position was weakened.^[12]

Hanan died on 24 July 1969, aged 60, after attending the annual conference of state attorneys general in Brisbane. He died in Cairns, Australia.^[2] His relatively early death is linked to his war injuries.^[5] He was buried at Invercargill's Saint Johns Cemetery.^[13] An act was passed to avoid the need for a by-election before the general election on 29 November, the 'By-election Postponement Act 1969'.^[14] His wife survived him by almost four decades and died on 26 July 2007; she is buried next to him.^[15]

• Gustafson, Barry (1986). The First 50 Years : A History of the New Zealand National Party. Auckland: Reed Methuen. ISBN 0-474-00177-6.
• Wilson, James Oakley (1985) [First ed. published 1913]. New Zealand Parliamentary Record, 1840–1984 (4th ed.). Wellington: V.R. Ward, Govt. Printer. OCLC 154283103.