Bahr was a member of the SPD from 1956 until his death in 2015. From 1960 to 1966 Bahr was the head of the Press and Information Office for the Land of Berlin (at that time, West Berlin). In that capacity, Bahr served as the spokesman for the Senate of Berlin, which was led at the time by Mayor Willy Brandt. From 1966 to 1969 Bahr served as an ambassador and as Ministerial Director of the Planning Staff of the German Foreign Office ("Auswärtiges Amt"). Bahr is considered to have been one of the most important and most influential advisors to Willy Brandt, especially with respect to the latter's policy of "Ostpolitik" (“Eastern Policy”, also known as “‘’Entspannungspolitik’’”, the German equivalent of "détente").