After his studies, De Jong started his career in 1979 at the Ministry of Foreign Affairs, where he worked as a diplomat at the NATO management.

From 1983 to 1987, De Jong worked at the Ministry of Social Affairs and Employment in the International Relations and European Integration Department.

From 1987 to 1993, De Jong subsequently worked at the Ministry of Justice and Security, where he was responsible as head of department for the development of the immigration and refugee policy.

There he wrote the Commission report, which for a long time was the basis for proposals in the field of asylum and migration policy of the European Union.

De Jong has been active in the Socialist Party (SP) since 2007, where he coordinated the working group on social globalization for several years.