Following the end of World War II Geertsema returned to the Leiden University graduating with a Master of Laws degree in July 1947.

Geertsema was elected as a Member of the House of Representatives after the election of 1959, taking office on 20 March 1959 serving as a frontbencher chairing the parliamentary committee for the Interior and the special parliamentary committee for Water Management in War Time and spokesperson for the Interior, Justice, Social Work, Provincial Government Affairs, Media, Kingdom Relations and deputy spokesperson for Civil Service and Local Government Affairs.

Toxopeus returned as Parliamentary leader on 12 March 1966 and Geertsema continued to serve in the House of Representatives as frontbencher again chairing the parliamentary committee for the Interior and spokesperson for the Interior, Justice, Provincial Government Affairs, Media, Kingdom Relations and deputy spokesperson for Social Work and Local Government Affairs.

Geertsema 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 (Heineken N.V., DSM Company, Rotterdam Dry Dock Company, NIBC Bank, Atlantic Association, SHV Holdings and Campina) and served on several state commissions and councils on behalf of the government (Public Pension Funds PFZW, Custodial Institutions Agency, Advisory Council for Spatial Planning, Raad voor Cultuur and KPN) and as an advocate, lobbyist and activist for LGBT rights and Social justice.

Geertsema was elected as a Member of the Senate after the Senate election of 1983, taking office on 13 September 1983 serving as a frontbencher chairing the parliamentary committee for the Interior and the parliamentary committee for Kingdom Relations and spokesperson for the Interior, Local Government Affairs, Provincial Government Affairs, Civil Service and Kingdom Relations.

In January 1987 Geertsema announced his retirement from national politics and that he wouldn't stand for the Senate election of 1987 and continued to serve until the end of the parliamentary term on 23 June 1987.