Douglas was born on 5 December 1937. His family had strong ties with the trade union movement, and was active in politics. Douglas was educated in Auckland, and gained a degree in accountancy from Auckland University.
Although he had some early experience with politics after serving on the Manukau City Council, his career in national politics began when he was elected to parliament as a Labour Party MP in Manukau. He began to become involved in the party's policies on industry and economics.
Labour, under Norman Kirk, won the 1972 elections, and Douglas was elevated to Cabinet. During Labour's term in office, he was Minister of Broadcasting, Minister of Housing, and Minister of Customs. However, Kirk's unexpected death in 1974 (and his replacement by the ineffectual Bill Rowling) led to Labour's defeat by the National Party under Robert Muldoon in 1975. Douglas became Labour's spokesperson on Housing until 1980, when he acquired responsibility for the Trade and Industry portfolio. In 1983, when David Lange became leader of the Labour Party, Douglas was made spokesperson for Finance.
In the elections of 1984, Labour was victorious. As Prime Minister, Lange gave the Minister of Finance role to Douglas. The policies that Douglas began to implement, however, were controversial. Known as "Rogernomics" (a coinage based on the term "Reaganomics, used to describe the economic policies of US President Ronald Reagan), the measures involved monetarist measures to control inflation, the slashing of subsidies and trade tariffs, and the privatization of public assets. All of these policies were regarded as a betrayal of Labour's left-wing policy platform, and were deeply unpopular with the public and ordinary party members. Douglas, along with his allies Richard Prebble and David Caygill, was able to gain enough backing within the party to continue, aided by Lange's reluctance to openly criticise his own ministers. Douglas's supporters defended the reforms as being necessary to revive the economy, which had been tightly regulated under National's Muldoon.
Labour was reelected in 1987, but this was primarily due to the weakness of the opposition National Party and to public support for Labour's other policies (such as its stand against nuclear weapons). There was little public backing for the reforms Douglas was implementing. Shortly after the election, Prime Minister Lange moved to stop the changes. A protracted conflict between Lange and Douglas broke out in Cabinet. Eventually, after Richard Prebble (a Douglas supporter) was dismissed, Douglas resigned, leaving his ministerial positions at the end of 1998.
The Labour Party caucus, however, voted to return Douglas to Cabinet in July the following year. While Lange was obliged to accept the decision of caucus, he was not required to give Douglas the exact same job. Douglas became Minister of Police and Minister of Immigration, much less senior positions than his old role as Minister of Finance. The rejection by caucus of Douglas's original dismissal, however, had weakened Lange's position, and the Prime Minister himself resigned the month after Douglas returned. Lange's successors (Geoffrey Palmer and Mike Moore) did not, however, restore Douglas to his former position, and Douglas was not able to pursue his economic program. Douglas resigned from parliament at the 1990 election (which Labour lost, mostly due to public anger at the economic policies that Douglas had implemented).
In 1993, Douglas was primarily responsible for creating the Association of Consumers and Taxpayers, intended to serve as a pressure group promoting the same economic policies that Douglas did. Shortly afterwards, the country switched to the MMP electoral system. (This change is sometimes attributed to public perceptions about betrayal by the established parties, with Labour's pursuit of non-leftist policies being the most major of those perceived betrayals). This meant that smaller groups had a much better chance of entering parliament, and encouraged the new association to establish the ACT New Zealand party. Douglas was the new group's first leader, but soon stood aside for Richard Prebble (his old ally from his days in the Labour caucus). Douglas has remained a strong supporter of the ACT party, although some say that he is unhappy with Prebble's alleged lack of focus on pure economic policy.
Since finally leaving politics when he surrendered leadership of ACT, Douglas has held senior positions at a number of prominent companies. He is now the managing director of his own group, Roger Douglas Associates.