Kühnert joined the SPD in 2005 and chaired the Jusos in Berlin from 2012 to 2015. Since 2015, he acted as deputy Jusos Federal Chairman and was responsible for tax policy, pension policy, structural policy, right-wing extremism and migration policy as well as for social media work. When Johanna Uekermann did not run as a candidate again, the Juso federal congress in Saarbrücken in November 2017 elected Kühnert as chairman of the Jusos, with 225 of 297 votes.