He is a Dutch politician and between July 1984 and August 1998 he was member of the European Parliament. During this period he was member of the Liberal and Democratic Group, member and Vice-Chair of the Liberal and Democratic Reformist Group, Chair of the Group of the European Liberal, Democrat and Reform Party, Chair of the Delegation for relations with Canada, member of the Committee on Economic and Monetary Affairs and Industrial Policy, member of the Delegation for relations with the Member States of ASEAN and the ASEAN Interparliamentary Organisation (AIPO), member of the Delegation for relations with the Member States of ASEAN and the Republic of Korea (AIPO), member of the Committee on External Economic Relations, member of the Delegation for relations with the United States, member of the Delegation to the EC-European Economic Area Joint Parliamentary Committee, member of the Committee on Legal Affairs and Citizens' Rights, member of the Delegation for relations with Central Asia and Mongolia.
He quit the European Parliament to become state secretary of the interior in the Dutch government Kok II. De Vries was the European Union's anti-terrorism coordinator from 25 March 2004 to March 2007. As of September 2008, he was the chairman of the European Security Research and Innovation Forum (ESRIF).