Following the end of World War II De Koster worked as a civil servant for the Diplomatic service of the Ministry of Foreign Affairs as an Attaché at the League of Nations in New York from June 1945 until April 1946.

In April 1977 De Koster announced that he wouldn't stand for the election of 1977 but wanted to run for the Senate and continued to serve until the end of the parliamentary term on 8 June 1977.

De Koster retired after spending 18 years in national politics and 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 (Randstad NV, Douwe Egberts, ASML Holding and the Atlantic Association) and serves on several state commissions and councils on behalf of the government (Adviesraad Internationale Vraagstukken) and as a diplomat and lobbyist for several economic delegations on behalf of the government and as an advocate for European integration.

Hans de Koster graduated from Higher Civic School and earned his bachelor's degree in economics in Amsterdam.

In the De Jong cabinet (1967-1971), he was State Secretary for Foreign Affairs on behalf of the People's Party for Freedom and Democracy (Dutch: Volkspartij voor Vrijheid en Democratie, VVD).

In 1972 de Koster presented the Rijckevorsel Commission report which proposed changes in the Dutch armed forces, including a major reduction in the army and transfer of Air Force roles to NATO partners, build a training area at Ter Apel and the first step towards a volunteer professional army.

From his personal archives, released in 2005, it was revealed that he breached the confidentiality of the private fixed Parliamentary Committee for Defence in June 1975, and the prince was informed about the developments.