Brash was born in the town of Wanganui, but his family moved to Christchurch when he was six. He received his education there, eventually gaining a BA in economics, history, and political science. He then continued his study in economics, receiving his MA in 1961. The following year, he began working towards a PhD at the Australian National University.
Brash's first entry into politics came in 1980, when he was asked to stand as the National Party's candidate for a by-election in the East Coast Bays electorate. Brash's attempt at the seat, however, was unsuccessful - some believe that this was a result of the decision by Robert Muldoon, National Party Prime Minister, to raise tolls on the Auckland Harbour Bridge, an important route for East Coast Bays. Brash contested the same seat in the general election of 1981.
In 1982, Brash was appointed to the position of managing director at the New Zealand Kiwifruit Authority, which oversaw the export of New Zealand kiwifruit. (Brash still grows kiwifruit as a hobby). Later, in 1986, he became general manager of Trust Bank, a newly established banking group.
In 1988, Brash became governor of the Reserve Bank of New Zealand, a position which he was to hold for the next fourteen years. Opinions are mixed regarding his performance there - supporters consider him to have been highly competent, while critics complain that he paid too much attention to strict inflation controls.
In April 2002, shortly before the general election, Brash resigned from his position to become a National Party candidate once again. Instead of contesting a electorate, however, Brash was made a list candidate, relying on MMP's proportional representation. Brash was placed fifth on National's list, a remarkable ranking for someone not already in parliament.
While National performed poorly in the election, Brash's high position on the party list enabled him to enter parliament. He was made National's spokesperson for financial issues, placing him opposite the Labour Party's Michael Cullen, the Minister of Finance. While Brash has been praised by some commentators for his knowledge about economics, some critics say that he has failed to develop any instinct for politics, and believe that he will not perform well until he has gained more experience.
In October 2003, Brash publicly challenged Bill English for leadership of the National Party. English, who was blamed by many for the party's poor performance since the election and for a number of policy gaffes, was unpopular with the party, but Brash, as a newcomer often criticised for lacking political talent, could not expect an automatic win because of this. In particular, Brash's decision to make his challenge public caused considerable criticism, as an open leadership dispute was perceived as highly damaging to the party's image. It was also seen as being a break with tradition, and (according to critics) an indication of Brash's "political naivety". Eventually, however, dissatisfaction with English coupled with the perceived policy-making competence of Brash enabled Brash to emerge the winner. Brash won a caucus vote on 28 October 2002, making him head of the National Party and Leader of the Opposition. He also remains National's finance spokesperson, but has said that he will surrender this role to newcomer John Key after the 2005 election.