Van Thijn attended the Amsterdams Lyceum from September 1945 until June 1952 and applied at the University of Amsterdam in June 1952 majoring in Sociology and obtaining an Bachelor of Social Science degree in June 1954 before graduating with an Master of Social Science degree in July 1958. Van Thijn worked as researcher at the Wiardi Beckman Foundation from August 1958 until February 1967. Van Thijn served on the Municipal Council of Amsterdam from May 1962 until September 1971. Van Thijn was elected as a Member of the House of Representatives after the election of 1967, taking office on 23 February 1967. After the election of 1972 the Leader of the Labour Party and Parliamentary leader of the Labour Party in the House of Representatives Joop den Uyl became Prime Minister in te Cabinet Den Uyl and Van Thijn was selected as his successor as Parliamentary leader in the House of Representatives on 11 May 1973. After the election of 1977 Den Uyl returned as Parliamentary leader on 8 June 1977 but he was still serving in the cabinet and because of dualism customs in the constitutional convention of Dutch politics he couldn't serve a dual mandate he subsequently resigned as a Member of the House of Representatives and Parliamentary leader on 8 September 1977 and Van Thijn again took over as Parliamentary leader on 8 September 1977. Following the cabinet formation of 1977 the Cabinet Den Uyl was replaced by the Cabinet Van Agt–Wiegel on 19 December 1977 and Den Uyl subsequently returned as a Member of the House of Representatives and as Parliamentary leader on 16 January 1978.