Favoured by such circumstances, he undertook to write the history of the reign of Manuel, and that of his predecessor and father, the emperor Calo-Joannes; and so well did he accomplish his task, that there is no history written at that period which can be compared with his work. The full title of this work is Ἐπιτομὴ τῶν κατορθωμάτων τῷ μακαρίτῃ βασιλεῖ καὶ πορφυρογεννήτῳ κυρίῳ Ἰωάννῃ τῷ Κομνηνῷ, καὶ ἀφήγησις τῶν πραχθέντων τῷ ἀοιδίμῳ υἱͅῷ αὐτοῦ τῷ βασιλεῖ καὶ πορφυρογεννήτῳ κυρίῳ Μανουὴλ τὧ Κομνηνῷ ποιηθεῖσα Ἰωάννῃ βασιλικῷ γραμματικῷ Κιννάμῳ. It is divided into six books, or more correctly into seven, the seventh, however, being not finished: it is not known if the author wrote more than seven books; but as to the seventh, which in the Paris edition forms the end of the sixth and last book, it is evidently mutilated. as it ends abruptly in the account of the siege of Iconium by the emperor Manuel in 1176. As Cinnamus was still alive when Manuel died (1180), it is almost certain that he finished the history of his whole reign; and the loss of the latter part of his work is the more to be regretted, as it would undoubtedly have thrown light on many circumstances connected with the conduct of the Greek aristocracy, and especially of Andronicus Comnenus, afterwards emperor, during the short reign of the infant son and successor of Manuel, Alexis II. In the first book Cinnamus gives a short and concise account of the reign of Calo-Joannes, and in the following he relates the reign of Manuel.

Possessed of great historical knowledge, Cinnamus records the events of his time as a man accustomed to form an opinion of his own upon important affairs; and, being himself a statesman who took part in the administration of the empire, and enjoyed the confidence of the emperor Manuel, he is always master of his subject, and never sacrifices leading circumstances to amusing trifles. His knowledge was not confined to the political state of the Greek empire ; he was equally well acquainted with the state of Italy, Germany, Hungary, and the adjoining barbarous kingdoms, the Latin principalities in the East, and the empires of the Persians and Turks. His view of the origin of the power of the popes, in the fifth book, is a fine instance of historical criticism, sound and true without being a tedious and dry investigation, and producing the effect of a powerful speech. He is, however, often violent in his attacks on the papal power, and is justly reproached with being prejudiced against the Latin princes, although he deserves that reproach much less than Nicetas and Anna Comnena. His praise of the emperor Manuel is exaggerated, but he is very far from making a romantic hero of him, as Anna Comnena did of the emperor Alexis. Cinnamus is partial and jealous of his enemies, rivals, or such as are above him; he is impartial and just where he deals with his equals, or those below him, or such persons and events as are indifferent to him personally. In short, Cinnamus shews that he was a Byzantine Greek. His style is concise and clear, except in some instances, where he embodies his thoughts in rhetorical figures or poetical ornaments of more show than beauty. This defect also is common to his countrymen; and if somebody would undertake to trace the origin of the deviation of the writers, poets, and artists among the later Greeks from the classical models left them by their forefathers, he would find it in the supernatural tendency of minds imbued with Christianism being in perpetual contact with the sensualism of the Mohammedan faith and the showy materialism of Eastern imagination. Xenophon, Thucydides, and Procopius were the models of Cinnamus; and though he cannot be compared with the two former, still he may be ranked with Procopius, and he was not unworthy to be the disciple of such masters. His work will ever be of interest to the scholar and the historian.