Under Emperor Alexius I Comnenus he held the offices of commander of the bodyguard and private secretary to the emperor, but in the succeeding reign he retired to Hagia Glykeria (one of the Princes' Islands), where he spent the rest of his life in writing books.
His most important work, Extracts of History (Επιτομή Ιστορίων), 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, whose first twenty books are not otherwise known. 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.).