Petersen was first elected into parliament at the 2011 election, where he received 3,662 votes. He served as Minister for Development Cooperation from 21 November 2013 to 3 February 2014, in the Thorning-Schmidt I Cabinet. When the Socialist People's Party left the coalition, a new cabinet was formed. Here Petersen served as Minister of Climate, Energy and Building from 3 February 2014 to 28 June 2015.[2][3]
In the 2015 election he received 2,151 votes and was not reelected. His election result did make him the Social Liberal Party's primary substitute in the Zealand constituency. He was called upon twice during the term: From 1 September 2015 to 31 December 2015 and from 6 September 2018 to 7 May 2019. Both times he were substitute for Zenia Stampe. Petersen ran again in the 2019 election, where he received 3,763 votes, getting elected into parliament again.[4][5]
