Mnesimachus was an Athenian comic poet of the Middle Comedy period, active in the 4th century BCE. No biographical details about his life survive. His floruit is generally placed in the mid-4th century, based on a fragment from his play The Horse-breeder that mentions the contemporary military commander Iphicrates.

He is known only through surviving fragments. The Suda attributes two plays to him: The Horse-breeder and The Busiris. The writer Athenaeus also cites a play titled Phaon. Approximately fifteen fragments survive, primarily preserved in Athenaeus’s Deipnosophistae. These offer glimpses of his style but are insufficient for reconstructing any plot.

Mnesimachus is a representative figure of the fragmentary record of Middle Comedy. His significance lies in the textual evidence his fragments provide for this transitional period. The fragments show characteristic themes of the era: The Horse-breeder parodies a military dispatch, while Busiris treats mythology. Common references to food and domestic life illustrate the period's broader shift toward social and everyday humor.