[6] In 2010, Brantner defended her PhD thesis "The reformability of the United Nations" at the University of Mannheim where she used to be a research associate at the department for Political Science II of Prof. Dr. Thomas König with a lectureship for International Policy.

In 2010, Brantner was (along with Richard Gowan) co-author of a study concerning the EU Human Rights Policy on behalf of the European Council on Foreign Relations.

During her studies at Sciences Po in Paris she founded a Green university group and was co-organiser of the first "European Students Convent" in 2001/2002.

In 2010, she joined the Friends of the EEAS, a unofficial and independent pressure group formed because of concerns that the High Representative of the Union for Foreign Affairs and Security Policy Catherine Ashton was not paying sufficient attention to the Parliament and was sharing too little information on the formation of the European External Action Service.

[9] In 2013, Brantner was elected to the Bundestag (the German Parliament) for Alliance '90/The Greens,[10] succeeding Fritz Kuhn as representative of the 274th district Heidelberg.

Between 2014 and 2017, Brantner served as chairwoman of the parliamentary Sub-Committee for Civilian Crisis Prevention and as member of the Committee on Family Affairs, Senior Citizens, Women and Youth.

[12] In the negotiations to form a coalition government under the leadership of Minister-President of Baden-Württemberg Winfried Kretschmann following the 2021 state elections, Brantner was part of the working group on European and international affairs, led by Theresa Schopper and Daniel Caspary.

[15] Since becoming a member of the Bundestag, Brantner has regularly abstained from parliamentary votes on the participation of Germany in United Nations peacekeeping missions as well as in United Nations-mandated European Union peacekeeping missions, including those for Afghanistan (2014), Somalia (2014, 2015 and 2018), Darfur/Sudan (2013, 2014), South Sudan (2013 and 2014) and the Central African Republic (2014).