In 1986 he succeeded Joop den Uyl as leader of the social democratic party Partij van de Arbeid. From 1989 until 1994 he was minister of financial affairs in a cabinet with the christian democratic party Christen Democratisch Appèl and the progressive liberal party D'66.
In 1994 he became Prime Minister in a cabinet with the liberal party Volkspartij voor Vrijheid en Democratie and D'66. After the elections in 1998 he led a second government with the same partners. His cabinet fell just weeks before the May 2002 elections when Kok and all his ministers stepped down because of the discussion about the possible Dutch responsiblity in the Srebrenica drama. After the elections he left politics, as he had already announced the year before.
See also: Politics of the Netherlands