In 1967, De Gaay Fortman left Amsterdam to become a senior lector at the University of Zambia in Lusaka, where he was head of the Economic faculty.

In 1968, he was appointed Chairman of the Agricultural Prices and Marketing Committee, an advisory Body of the Zambian government.

While in Zambia, De Gaay Fortman kept close tabs on the developments in Dutch politics.

In 1971, De Gaay Fortman returned to the Netherlands from Zambia for the PPR's campaign for the 1971 general election.

During his period in the House of Representatives, he was Professor Extraordinary of Economic Development at the Institute of Social Studies in The Hague.

Before 1977 general election, the younger Ria Beckers succeeded De Gaay Fortman.

He also held many positions in the World of Development Cooperation: most importantly he was president of NOVIB, the Dutch branch of Oxfam, between 1977 and 1984.

In 1992, he served as the chair of the Joint Steering Committee of the Netherlands Israeli Palestinian Research Programme (NIRP).

Until 2005, he taught his trademark class "Political Economy of Human Rights", which was part of the Master in Conflict Studies at University of Utrecht.

In 2002 he also became a member of the permanent committee for Development Cooperation of the Advisory Council for International Questions of the Minister of Foreign Affairs.

From 2003 to 2004 he was the Msgr Willy Onclin Professor of Comparative Canon Law at the Catholic University of Louvain.

His father was Gaius de Gaay Fortman, a prominent Dutch politician who served as Minister of the Interior.

W. F. and B. de Gaay Fortman served as members of the Senate together, but representing different political parties.