From
2009 to 2019, Andresen served as a member of the
Landtag of Schleswig-Holstein, representing
Flensburg. At the time of his first election, he was the parliament's youngest member at 23. From 2017 until 2019, he served as vice-president of the Landtag. Andresen has been a
Member of the European Parliament since the
2019 European elections. He has since been serving on the
Committee on Budgets. He is also his parliamentary group's coordinator on the committee. In addition to his committee assignments, Andresen is part of the Parliament's delegation for relations with
Canada. He is also a member of the European Internet Forum, the European Parliament Intergroup on Anti-Corruption, the European Parliament Intergroup on Anti-Racism and Diversity, the
European Parliament Intergroup on LGBT Rights, the European Parliament Intergroup on Seas, Rivers, Islands and Coastal Areas and the European Parliament Intergroup on Traditional Minorities, National Communities and Languages. Since 2021, Andresen has been leading the German delegation within the
Greens–European Free Alliance group, succeeding
Sven Giegold. ==Other activities==