Demetrios was the son of Matthew Kantakouzenos, governor of Morea, and Irene Palaiologina.
Demetrios was given the title of sebastokrator by Emperor John V Palaiologos in December 1357 and went to the Peloponnese with his father and grandfather in 1361.
Our only information for this event is a cryptic reference in the Emperor Manuel II Palaiologos' funeral oration for his brother Theodore, who remarks on the insubordination of the "son" of Matthew Kantakouzenos, who had usurped the government on the death of Manuel Kantakouzenos in 1380.
Zakythenos, a historian of the Despotate of the Peloponnese, was inclined to believe that the son was Demetrios.
[1] According to the Byzantinist Donald Nicol, "This problem can hardly be satisfactory solved on the basis of the documentary evidence available".