Birkner was elected to the
State Parliament of Lower Saxony in the
2008 state elections. However, in February 2008 he left the Landtag to become a State Secretary for the State Minister of the Environment in the
second cabinet Wulff,
Hans-Heinrich Sander. He was replaced by
Christian Grascha. From January 2012 until 2013, he served as State Minister for Environment, Energy and Climate Protection in the government of Minister-President
David McAllister. In September 2017, Birkner succeeded
Christian Dürr as chairman of the FDP's parliamentary group. In the negotiations to form a so-called
traffic light coalition of the
Social Democrats (SPD), the
Green Party and the FDP following the
2021 federal elections, Birkner led his party's delegation in the working group on environment policy; his co-chairs from the other parties were
Rita Schwarzelühr-Sutter (
SPD) and
Steffi Lemke (
Bündnis 90/Die Grünen). Birkner was nominated by his party as a delegate to the
Federal Convention to elect the
President of Germany in
2022. ==Life after politics==