He received his doctorate in 1966 and habilitated four years later. He has been with Gleiss Lutz as Of Counsel since 2005. Appointment as professor of public law, initially at the Free University of Berlin in 1972. Rupert owned the Chair of Public Law, in particular administrative science and financial law at the University of Munich from 1978 on. He has been on emeritus status since October 2005. From 1981-1988 he was the senator of the state of Berlin for legal, federal and European affairs. One year later he was the federal minister of defence. From 1990–2002 Rupert was a member of the Bundestag; 1998–2002 chairman of the legal affairs committee of the Bundestag.

Rupert is a member of various legal and economic associations, foundations, advisory and supervisory boards.

He speaks German and English.