In 1983 he left that role and moved to the position of Program Director for Western Europe and the Near and Middle East at the Social Science Research Council, New York, a post he held until 1988.

From 1988 to 1991 Diamandouros was Director of the Greek Institute for International and Strategic Studies, a research organisation in Athens.

From 1988 to 1996 he also returned to the Social Science Research Council, this time in the role of co-chair of the Subcommittee on Southern Europe.

He is also joint General Editor of an Oxford University Press series on New Southern Europe and has received research grants from Fulbright and the National Endowment for the Humanities.

On April 1, 2003, he was elected Ombudsman for the European Union to serve out the remainder of Söderman's term.