From 1993 to 1994, he served as Foreign Minister of the Netherlands, succeeding Hans van den Broek.

His father was Johannes Kooijmans, an engineer and a member of the municipal council of Heemstede, and his mother was Alida Jonker.

[2] Kooijmans went to the secondary school Eerste Christelijk Lyceum in Haarlem, where he followed the gymnasium program in humanities.

After the election of 1972 Kooijmans was appointed as State Secretary for Foreign Affairs in the Cabinet Den Uyl until 19 December 1977.

The Cabinet Den Uyl fell on 22 March 1977 after four years of tensions in the coalition and continued to serve in a demissionary capacity.

Kooijmans also served as a diplomat on behalf of the United Nations as a special rapporteur on Human rights and Torture.