[1] Westerterp attended a gymnasium in Rotterdam from April 1943 until May 1949 and applied at the Radboud University Nijmegen in June 1949 majoring in Political science, obtaining a Bachelor of Social Science degree in 1951 before switching to Journalism and graduating with a Master of Arts degree in Journalism in July 1953.

After the 1971 general election, Westerterp was appointed State Secretary of Foreign Affairs in the Biesheuvel I cabinet, taking office on 17 August 1971.

After the 1972 general election, Westerterp returned to the House of Representatives, taking office on 7 December 1972 but he was still serving in the cabinet and because of dualism customs in the constitutional convention of Dutch politics he could not serve a dual mandate he subsequently resigned as State Secretary of Foreign Affairs on 7 March 1973.

He took many measures that had a lasting positive impact on traffic safety in the Netherlands, like making seat belts and the moped helmet mandatory.

Westerterp also became active in the private sector and public sector and occupied numerous seats as a corporate director and nonprofit director on several boards of directors and supervisory boards (DSB Bank, Radio Netherlands Worldwide, DSM Company, Van Lanschot, Atlantic Association, Randstad NV and the Institute of International Relations Clingendael) and served on several state commissions and councils on behalf of the government (Public Pension Funds APB, Cadastre Agency and the Advisory Council for Foreign Affairs) and as an advocate and lobbyist for Highway engineering improvements and European integration and as a political consultant for the Livable Netherlands (LN) party.