He was elected to Parliament in 2005 as a list MP for the National Party and held the Ōtaki electorate from 2008 until 2020, when he retired.

He narrowly lost the election to Labour's Darren Hughes, by a margin of 1.00% or 382 votes[10] but entered Parliament as a list MP.

[11] When John Key became National leader in 2006, Guy became the party's junior whip and an associate spokesperson for agriculture.

[14] Guy held the electorate three subsequent times, with his greatest majority being 7,782 votes over the Labour candidate in 2014.

[15][16][17] On 15 June 2009 Guy was selected as the new Minister of Internal Affairs, a position outside of the Cabinet, to replace Richard Worth after the latter resigned following allegations of sexual harassment.

Guy lost his ministerial roles when the National Party was not returned to government at the 2017 general election.

[22] From March 2018 until his announcement on 30 July 2019 that he would not seek re-election in 2020, he was the spokesman for agriculture, biosecurity and food safety.

[26][27] In November 2020, Guy contested but failed to win a seat on the board of New Zealand dairy co-operative Fonterra.