Richardson was elected to Parliament in 1981, standing in the Christchurch electorate of Selwyn. She was a member of the governing National Party, then led by Prime Minister Robert Muldoon. When National lost the 1984 election, Richardson became a member of the Opposition.
Richardson initially stood out in National's caucus for her strong support of the radical economic reforms of the Labour Party's Finance Minister, Roger Douglas. These reforms, sometimes known as "Rogernomics", involved the privatization of state assets, the removal of tariffs and subsidies, and the control of interest rates. These reforms were against the traditional policies of the left-wing Labour Party, but were also opposed by the more conservative wings of the National Party. Particularly hostile were followers of Robert Muldoon, a traditionalist conservative who opposed free market reforms as undermining state authority.
Shortly after National's electoral loss, Jim McLay replaced Muldoon as leader of the National Party, and there was a considerable rearrangement of responsibilities. People such as Bill Birch and George Gair, who McLay associated with the Muldoon era, were demoted. They were replaced by newer MPs, such as Richardson and Simon Upton, who McLay believed would help revitalize the party. This move proved fatal to McLay personally, however, as the sacked Birch and Gair allied themselves with McLay's rival, Jim Bolger. Bolger ousted McLay and became party leader.
The change in leadership was damaging for Richardson, as Bolger (and many of his allies) strongly disliked her. This dislike was due to three main factors: anger at McLay's "favouritism" towards her, dislike of her radical economic policies, and dislike of her personality (which many colleagues found "abrasive" and "condescending"). When George Gair (elevated for his role in Bolger's rise to power) retired from the position of deputy leader, Richardson stepped forward for the position. Bolger, however, made it clear that he strongly opposed Richardson's candidacy, instead throwing his support behind Don McKinnon. McKinnon defeated Richardson and became deputy leader.
Bolger did, however, make Richardson the party's spokesperson on finance. This was an attempt to pacify Richardson and her supporters, rather than an expression of confidence in her - it was well known that Bolger himself preferred the more cautious Bill Birch for the finance role. The move to defuse tension was only partially successful, and hostility between supporters of Bolger and supporters of Richardson remained. Many National politicians believed that Richardson sought to replace Bolger as leader, but even if Bolger was vulnerable, the two factions that opposed him (one led by Richardson and the other led by Winston Peters) were unwilling to cooperate. Bolger's leadership remained secure, and when his popularity rose, the window of opportunity was lost.
When National came to power in the 1990 elections, Richardson had enough support within the party to be made Minister of Finance, a role Bolger would rather have given to Bill Birch. Many people, having been told by National that the new government would adopt more cautious, conservative policies than the radical Labour government, were disapointed when Richardson continued the policies established by Douglas. As a result of the policies, Richardson became one of the most disliked politicians in the country. Her first Budget, which introduced severe cuts to social welfare, compounded this unpopularity.
Although National gained reelection in the 1993 elections, many people within the party believed that Richardson's presence was damaging to them. In addition, Bolger and his allies had still not been reconciled with her. In 1994, Richardson lost her role as Minister of Finance, being relegated to the backbenches. She was replaced by Bill Birch, Bolger's original preference.
Ruth Richardson left parliament the same year, although continued to be involved in politics through her advocacy of the ACT New Zealand party. ACT, established by Roger Douglas and his allies, promotes policies very close to those of Richardson, and in late 2003, it was reported that ACT was attempting to convince her to stand as one of their candidates in the 2005 elections. She has also a number of roles related to business and corporate governance, and served on a number of corporate boards.