[1] From 1967 to 1969, Cullen taught English at Saint Edwards School, Florida,[1] and joined Ford Motor Company Limited in 1969, where he worked in international media relations.
[12] From 1996 until 1999, Cullen served as senior advisor to the vice presidency of the World Bank's external and UN affairs department.
[1][13] Cullen played an active role in the World Bank's work on the reconstruction of Bosnia[14] and the transformation of the former Soviet Union countries into market economies.
[21] He was involved in the creating the World Commission on Dams and co-ordinated the launch of its final report in November 2000.
[38] In 2010, Cullen visited Pyongyang where he taught negotiation to the United Nations' North Korea team.
[38] Between 2010 and 2017, he contributed to WUCF-TV's Global Perspectives where he discussed the challenges faced by small countries,[42][43] effective negotiation techniques for politics,[44] and doing business with China.
[45] In 2016 and 2017, Cullen made appearances on the BBC Radio 4 programme The Bottom Line as a trade deal expert.