Carter stood in the electorate in the as a successor to Gail McIntosh, but was defeated by Labour's Ruth Dyson. Carter was first elected to Parliament in the 1994 by-election in Selwyn, replacing the resigning Ruth Richardson. In the 1996 general election he won the Banks Peninsula electorate against Dyson. In the 1999 election he was defeated by Dyson, but entered Parliament as a list MP. In the 2002 election, he failed to recapture the seat and remained a list MP. From 1998 until the National Party's defeat in 1999 Carter was Minister for Senior Citizens, Associate Minister of Revenue, and Associate Minister for Food, Fibre, Biosecurity and Border Control. At the very end of National's term in office, he was also Associate Minister of Education. In 2008, Carter was initially chosen as the National candidate for the resurrected safe National seat of Selwyn, but opposition to this saw the National candidacy up for grabs again. He pulled out and the candidacy was eventually won by Amy Adams, who won the seat. Carter was given a high list placing of nine instead and did not contest an electorate. After National's election victory, he took the portfolios of Agriculture, Biosecurity and Forestry.
Minister of Agriculture
In May 2010, Carter issued a ban on kosher slaughter, rejecting the recommendations of his advisers. Carter held shares in a firm that exports meat, and prior to instituting the ban he met senior managers of the firm who wanted a ban on kosher slaughter to reduce their competition.
Minister of Primary Industries
After the 2011 election, Carter was appointed Minister of the new Ministry of Primary Industries. In November 2012 he approved the increased squid fishery SQU6T by 140%, despite recommendations from scientists and the Department of Conservation that this would be detrimental to the endangered New Zealand sealion.
On 22 January 2013, the Prime Minister John Key announced that Carter was his preference to replace Lockwood Smith as Speaker of the House. Carter's appointment was not without controversy, and the Labour Party questioned whether he actually wanted the job. As the opposition was not consulted as per convention, Trevor Mallard was nominated by Labour and the position was put to a vote on 31 January 2013. Carter won by 62 votes to 52. Consistent with the tradition of newly elected speakers, Carter had to be "dragged to the chair" following the election. The office of speaker entitles Carter to the title The Right Honourable following a reform of the New Zealand royal honours system in 2010. Carter cited his intention to continue as Speaker, "if that is the will of Parliament", as the basis for his decision to stand as a list-only candidate in the 2014 general election. On 10 November 2015, Carter controversially failed to acknowledge offence caused to significant numbers of Labour and Green MPs after John Key had accused them of "backing rapists" during a debate about the Christmas Island Detention Centre. The following day, Carter silenced seven female MPs who stated that they were victims of sexual abuse and stood up to express personal offence to Key's statement, which they called on Key to apologise for. Carter ruled that the manner in which they stood to address the house was contrary to the House's standing orders and dismissed several of the seven; the remainder walked out. On 11 May 2016, Carter dismissed the Prime Minister, John Key, from the debating chamber. Key had ignored several of the Speaker's warnings about behaviour contrary to the standing orders. Carter stated, "He is to be treated no differently to any other in this house". Carter was returned as an MP from the National Party list in the 2017 election. Following the election a Labour Government was appointed; Carter was replaced as Speaker by Labour's choice, Trevor Mallard, who was elected on 7 November 2017.
