He returned to Mainz for his "Referendariat" in 1985, working also as an assistant to Eckart Klein, at the Institute for International and European Law.
In the summer semester of 1987 he studied as legal clerk (Rechtsreferendar) at the German University of Administrative Sciences Speyer.
He spent his research sabbatical as visiting fellow at the Lauterpacht Centre for International Law at the University of Cambridge in 2007.
In 2009, Thomas Giegerich gave a lecture at the 69th Annual Assembly of the "Vereinigung der Deutschen Staatsrechtslehrer" in Graz.
As co-opt member of the executive board, he had organized the 72nd Annual Assembly that took place in October 2012 in Kiel.