Brunner has been a member of the German Bundestag since 2013. He is a member of the Bundestag Defence Committee, the Committee on Legal Affairs and Consumer Protection, and serves as Spokesperson for the Subcommittee on Disarmament, Arms Control, and Non-Proliferation. On the Defence Committee, he serves as the rapporteur of his parliamentary group for the German Air Force.
He worked at the Kempten (Allgäu) local court and public prosecution office from 1978 to 1990, and then served as mayor of the town of Illertissen from 1990 to 2002. He joined the Social Democratic Party of Germany (SPD) in 1982 and served as chairman of the SPD parliamentary group on the council from 1998 to 2012, as well as chairman of SPD electoral district 255 (Neu-Ulm, Günzburg, Unterallgäu) from 1998 to 2009. He is also a member of the Social Democratic Party of Austria and the Czech Social Democratic Party.
As a member of the NATO Parliamentary Assembly, Brunner joined the election observation mission for the 25 May 2014 early presidential election in Ukraine during the conflict in Donbass.