The contemporary historian Niketas Choniates describes him as a brave, audacious and experienced soldier but frequently led astray by his foolhardiness and presumption.
[1] He was one of the victims of the reign of terror unleashed by the usurper Andronikos I Komnenos (reigned 1183–1185) in 1183, who had him blinded, though possibly not completely, then thrown into prison, because he attacked a court eunuch who blamed the disasters befalling the empire on the incompetence of Andronikos' predecessor, his under-age nephew Alexios II Komnenos (r. 1180–1183).
[2][1] Donald Nicol argues this shows he was a partisan of the House of Angelos, and he had his reward when Isaac II Angelos (r. 1185–1195, 1203–1204) came to the throne in 1185: the new emperor give him the rank of Caesar and appointed him to command the expedition against the Bulgarians in 1186, who had recently rebelled against the Byzantine rule and had restored the Bulgarian Empire.
The marriage was first disallowed by the Patriarch of Constantinople, Luke Chrysoberges, and the then-reigning emperor, Manuel I Komnenos (r. 1143–1180), because a seventh degree of consanguinity existed between John and Irene.
[5][6] John's parents are not known, but he may have been brother to Theodore Kantakouzenos (died 1184), whom Niketas Choniates states was killed assisting Isaac Angelos defend the city of Nicaea against Andronikos Komnenos.