The prominent expert on Soviet and GDR politics and history passed away on Sunday in hospital in Manderscheid in Rhineland-Palatinate after a long illness.
He is survived by his wife Elke and a son from his previous marriage, and leaves behind a unique library of 6,000 books on the Soviet Union and the GDR.
Born in Vienna and originally named Vladimir, Leonhard went to live Russia in 1935 with his Communist-inspired mother. She along with thousands more foreigners who came to the Soviet Union to help build the workers' utopia was sent to a Soviet labour camp for 12 years during Stalin's purges.
After the end of the Second World War in 1945, the young Marxist-Leninist moved to Germany as the youngest member of the so-called Ulbricht Group of German-born Communists.
Led by Walther Ulbricht, who founded the GDR, they were assigned to organize the administration of the Soviet occupation zone.
But shortly before the creation of the Soviet-style East German state, Leonhard said his eyes were opened by a five-hour speech by Ulbricht outlining his intent to set up a "bureaucratic dictatorship".
"By the summer of 1949, it became clear to me," he told Die Welt newspaper in 2007. "We would become a province of the Soviet Union under Stalin. That meant I belonged to the opposition."
Leonhard soon fled to Yugoslavia and then moved to the newly founded West Germany, where he became a key figure for the dissident movement.
He chronicled his transformation from zealous Communist to system opponent in his 1955 work "Children of the Revolution", before going on to study history at the Oxford and Columbia universities.
Leonhard then established himself as a prominent expert on the Soviet Union and the GDR and as a vehement critic of the Soviet system.
He also lectured for more than two decades at Yale University until 1987.