Dahmen joined Alliance 90/The Greens in 1998. From 2010 until 2018, he was part of the party's leadership in North Rhine-Westphalia, under co-chairs Sven Lehmann and Mona Neubaur.
Dahmen became a member of the Bundestag in 2020 when he replaced Katja Dörner who had resigned.[3] In parliament, he has since been serving on the Committee on Health.[4] Since 2021 he has been his parliamentary group's spokesperson on health policy. He is also the group's rapporteur on digital health.[5]