Mnesimachus of Phaselis is known solely from a brief mention by the Byzantine scholar Photius, who identifies him as a historian from the Greek colony of Phaselis in Lycia. No biographical details survive. Scholars place him in the 4th century BCE based on the subject of his work and his position in such catalogs.

His only attested work is a History of Alexander, which is now completely lost. Photius records that it was composed in the Ionic dialect.

Mnesimachus’s significance lies in his status as one of the many lost historians of Alexander the Great. His use of the Ionic dialect connects him to the older historiographical tradition of Herodotus. He represents the fragmentary nature of the primary source material for Alexander’s reign and is cited in modern catalogs as a minor, lost source.