Kox studied law at Tilburg University. From 1975 to 1978 he worked in Tilburg law centre, in 1978 at the municipality of Oosterhout, and from 1978 to 1982 again in Tilburg law centre. From 1981 to 1994 he was editor-in-chief of Tribune, SP’s party magazine, and from 1982 to 1999 he was SP fraction leader of the municipal council of Tilburg. Furthermore he has been exercising several jobs in the party executive.
Since June 2003 Kox has been a member of the Senate. He is also chairman of the Party of the European Left in the Parliamentary Assembly of the Council of Europe and an employee of SP’s policy institute.