Mario Keßler (born 4 May 1955) is a German historian. He was born in what was then the German Democratic Republic (East Germany). He was 34, and about to finish his habilitation (senior level university qualification) by the time the wall was breached. German reunification thesis, in 1990, transformed the historiographical context on both sides of the former inner German border, but the changes were particularly stark for scholars who had learned their profession in the east. By the mid-1990s only around 40 East German professional historians were still in academic employment and by 2017 only around ten East German trained historians were employed by universities. Mario Keßler was one of these few historians who made the professional transition to the post-reunification world successfully.