Siegesmund joined Alliance 90/The Greens in 2002, and from 2003 to 2008 was a consultant in the office of Bundestag deputy Katrin Göring-Eckardt. From 2004 to 2012 she was the spokeswoman of the Jena district association of the Greens.[1]
In the 2009 state election, she was elected to the Landtag of Thuringia and became leader of the Greens parliamentary group. She was one of the Greens' two top candidates in the 2014 state election, alongside Dirk Adams, and was re-elected to the Landtag.[3] In December, she resigned as group leader to become Minister for Environment, Energy, and Nature Conservation in the first Ramelow cabinet. She was one of two Greens ministers in cabinet, alongside Dieter Lauinger. In 2015, she was given the additional portfolio of Second Deputy Minister-President.[1] On 15 October 2015, citing separation between executive and legislature, she resigned from the Landtag.
In the 2019 Thuringian local elections, Siegesmund was elected to the city council of Jena.[4][5]
Siegesmund again served alongside Dirk Adams as the Greens' top candidates in the 2019 state election, and was elected as the number one candidate on the party list.[6] She ran as a candidate in the constituency Jena I and placed second with 24.7% of the vote, the best performance of any Greens candidate in the election.[7]
Siegesmund left cabinet after the defeat of the government during the February 2020 Thuringian government crisis, but returned after Bodo Ramelow was re-elected in March.[1] On 17 March, she again resigned her seat in the Landtag.[8]