He is the brother of Morten Helveg Petersen, who serves as a member of the European Parliament for the same party.

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.

His election result did make him the Social Liberal Party's primary substitute in the Zealand constituency.