Under Emperor Alexios I Komnenos he held the offices of commander of the bodyguard and private secretary (protasēkrētis) to the emperor, but after Alexios' death, he retired to the monastery of St Glykeria, where he spent the rest of his life in writing books.
His most important work, Extracts of History (Greek: Ἐπιτομὴ Ἱστοριῶν, Latin: Epitome Historiarum), in eighteen books, extends from the creation of the world to the death of Alexius (1118). The earlier part is largely drawn from Josephus; for Roman history he chiefly followed Cassius Dio up to the early third century. Contemporary scholars are particularly interested in his account of the third and fourth centuries, which depend upon sources, now lost, whose nature is fiercely debated. Central to this debate is the work of Bruno Bleckmann, whose arguments tend to be supported by continental scholars but rejected in part by English-speaking scholars. An English translation of these important section has recently been published: Thomas Banchich and Eugene Lane, The History of Zonaras from Alexander Severus to the Death of Theodosius the Great (Routledge 2009). The chief original part of Zonaras' history is the section on the reign of Alexios Komnenos, whom he criticizes for the favour shown to members of his family, to whom Alexios entrusted vast estates and significant state offices. His history was continued by Nicetas Acominatus.
Various ecclesiastical works have been attributed to Zonaras — commentaries on the Fathers and the poems of Gregory of Nazianzus; lives of Saints; and a treatise on the Apostolical Canons — and there is no reason to doubt their genuineness. The lexicon, however, which has been handed down under his name (ed. J. A. H. Tittmann 1808) is probably the work of a certain Antonius Monachus (Stein's Herodotus, ii. 479 f.).