He was a member of the Council of State of the Netherlands from 1976 to 1990 and subsequently served as judge at the European Court of Justice from 1990 until 2000.

Een proeve in Europees Parlementarisme and supervised by Frederik Mari van Asbeck,[3] was about the Common Assembly of the European Coal and Steel Community between 1952 and 1958, in which his father served during that period.

[4][5] He only worked shortly at Leiden University as he was appointed member of the Council of State of the Netherlands per 29 November 1976.

Kapteyn was President of the Administrative Jurisdiction Division of the Council of State between 1985 and 1989, in which he was succeeded by Piet Boukema.

After his return to the Netherlands he served as professor of European Studies (Ynso Scholten professorship) at the University of Amsterdam between 2000 and 2005.