Rücker previously served as a foreign policy adviser to the Social Democratic parliamentary group in the German Bundestag.

Rücker also held various postings in the Federal Foreign Office in Bonn and German embassies abroad, including Dar es Salaam, Detroit (as Consulate-General) and Vienna.

From September 2006 to 20 June 2008, Rücker served as the Special Representative of the United Nations Secretary-General for Kosovo and the head of UNMIK.

[1] On July 3, 2015, Rücker over-ruled the election of Katrin Nyman-Metcalf, the Estonian candidate ranked first by a "consultative group" of five ambassadors – from Poland, Chile, Greece, Algeria and chaired by Saudi Arabia – for the office of the United Nation's first special rapporteur on digital privacy after activist groups said she would not be a strong enough critic of U.S. surveillance.

[2] Germany and Brazil, whose governments were targets of U.S. surveillance, were the original initiators of the investigator post, approved unanimously by the 47 Council members in March 2015.