Boertien became a Member of the House of Representatives after Jan Smallenbroek was appointed as Minister of the Interior in the Cabinet Cals following the cabinet formation of 1965, taking office on 18 May 1965 serving as a frontbencher and spokesperson for Justice, Law enforcement, Transport, Postal Services, Medical Ethics, Gambling and deputy spokesperson for Social Affairs and the Royal Family.

Boertien was selected as a Member of the European Parliament and dual served in those positions, taking office on 8 May 1967.

After the election of 1971 Boertien was appointed as Minister for Development Cooperation in the Cabinet Biesheuvel I, taking office on 14 July 1971.

After the election of 1972 Boertien returned as a Member of 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 couldn't serve a dual mandate he subsequently resigned as a Member of the House of Representatives on 7 March 1973.

Boertien subsequently returned as a Member of the House of Representatives after Antoon Veerman was appointment as State Secretary for Education and Sciences in the new cabinet, taking office on 28 May 1973 serving as a frontbencher chairing the parliamentary committee for Kingdom Relations and the special parliamentary committee for European Parliamentary Reforms and spokesperson for Foreign Affairs, Defence, European Affairs, Benelux Union and deputy spokesperson for Medical Ethics and Abortion.