State politics Müller was elected to the
Berlin House of Representatives in 1996 and has been a member ever since. On 16 June 2001, he took over leadership of the
SPD Group in the
House of Representatives from Klaus Wowereit who had been elected Governing Mayor of Berlin. After the resignation of Peter Strieder in April 2004, Müller ran for the Berlin SPD State Party Leadership and won, remaining the Leader of Berlin SPD until he lost his re-election bid against Jan Stöß in June 2012. Following the
2011 state elections in Berlin, Müller was senator for urban development and the environment from 1 December 2011 to 11 December 2014. One of his most extensive projects, which planned to build business space and apartments on the grounds of the former
Tempelhof Airport, was rejected in 2014 by a clear majority of Berlin citizens in a referendum.
Mayor of Berlin, 2014–2021 in Berlin In December 2014, Müller took over in midterm from the incumbent
Klaus Wowereit, Berlin's longest-serving mayor since reunification in 1990, who presided over the city's emergence as a popular tourist destination and center for high-tech start-ups but stepped down after his popularity suffered from the debacle around the
Berlin Brandenburg Airport. Müller had previously made a surprise entry into the race to succeed Wowereit and defeated two candidates initially seen as the most likely successors. In the negotiations to form a so-called
traffic light coalition of the SPD, the
Green Party and the
Free Democrats (FDP) on the national level following the elections, he was part of his party's delegation in the working group on innovation and research, co-chaired by
Thomas Losse-Müller,
Katharina Fegebank and
Lydia Hüskens. In parliament, Müller served on the Committee on Foreign Affairs (2021–2025) and its Subcommittee on Foreign Cultural and Educational Policy (2022–2025). He also chaired a study commission set up to investigate the entire period of German involvement in Afghanistan from 2001 to 2021 and to draw lessons for foreign and security policy in future. In addition to his committee assignments, Müller chaired the German-Japanese Parliamentary Friendship Group. ==Other activities==