Studies and early career

Kohnstamm attended a Gymnasium in The Hague from April 1962 until May 1968 and applied at the University of Amsterdam in June 1971 majoring in Law and obtaining an Bachelor of Laws degree in June 1973 and worked as a student researcher before graduating with an Master of Laws degree in July 1977. Kohnstamm worked as a criminal defense lawyer in Amsterdam from October 1977 until June 1981.

House of Representatives

Kohnstamm was elected as a member of the House of Representatives after the election of 1981, taking office on 10 June 1981 serving as a backbencher. After the election of 1982 Kohnstamm was not reelected and he continued to serve until the end of the parliamentary term on 16 September 1982. Kohnstamm again worked as a criminal defense lawyer in Amsterdam from September 1982 until June 1986. Kohnstamm served as Chairman of the Democrats 66 from 30 October 1982 until 20 May 1986.

After the election of 1986 Kohnstamm returned as a member of the House of Representatives, taking office on 3 June 1986 serving as a frontbencher chairing the parliamentary committee for Law enforcement and parliamentary committee for the Ombudsman and spokesperson for the Interior, Justice, Law enforcement, Health, Civil Service and Abortion and deputy spokesperson for Foreign Affairs, European Affairs and the Benelux Union.

State Secretary for the Interior

After the election of 1994 Kohnstamm was appointed as State Secretary for the Interior under the first cabinet of Prime Minister Wim Kok, taking office on 22 August 1994. He was tasked with public security, emergency services, emergency management and urban planning. In December 1997 Kohnstamm announced that he would not stand for the election of 1998. Following the cabinet formation of 1998, Kohnstamm asked not to be considered for a post in the new cabinet; the First Kok cabinet was replaced by the Second Kok cabinet on 3 August 1998.

Late career

Kohnstamm remained in active in national politics; he was elected as a member of the Senate after the 1999 election, taking office on 8 June. He served as a frontbencher and chaired the special parliamentary committee for the European Court of Justice and spokesperson for Foreign Affairs, Justice, Health, European Affairs, Art and Culture. He left the Senate on 7 September 2004. He was the chair of the Dutch Data Protection Authority from 2004 to 2016. From 2016 tot 2021 he was the chair of the Euthanasia Review Committee. From 2021 to 2023 he chaired the Restitution Committee that gives recommendations and binding opinions concerning Nazi looted art. He also was chair of board of the International Chamber Music Festival Schiermonnikoog (2012-2021) and of the board of the foundation Forbidden Music Regained (2014-2021)