He was a senior adviser to Tasmanian premier Robin Gray from 1988 to 1989 and was later managing director of Guy Barnett and Associates, a political and public relations consultancy, from 1989 to 2002.

Barnett was appointed to the Senate on 26 February 2002 to fill a casual vacancy caused by the resignation of Brian Gibson.

[1] In a November 2008 preselection ballot, Barnett was placed in third position on the Liberal Party's Senate ticket in Tasmania for the next federal election, behind Eric Abetz and Stephen Parry.

The result was widely reported as a demotion, given the party had only secured two Senate seats in Tasmania at the previous election.

[8] Barnett served as Minister for State Development, Construction and Housing under Jeremy Rockliff from April 2022 to July 2023.

In October 2023, following a ministerial reshuffle caused by the resignation of Elise Archer, he was additionally appointed Attorney-General of Tasmania, at which point he relinquished the Energy and Renewables portfolio.

[12] In June 2008, Barnett moved a motion in the Senate to end Medicare funding for abortions occurring after the second trimester, stating that there should be "compassion and care for the unborn, we want to show love and concern for them rather than killing them in many cases for psychosocial reasons, which is effectively abortion on request".