In 358 BC, Mentor, along with his brother Memnon, were hired to provide military leadership by a rebel Persian satrap, Artabazus. Despite Mentor's capable leadership, the rebellion failed, and Artabazus, Barsine and Memnon fled to Macedon, where they were welcomed by Phillip II. Mentor fled to Egypt.

Pharaoh Nectanebo II immediately enlisted the aid of the Greek mercenary, as he expected a Persian invasion was imminent. The pharoah sent Mentor, at the head of 4000 mercenaries, to support Sidon, which had rebelled from Persia. Although Mentor won significant victories against some of the satraps, he was unable to defeat Artaxerxes' army, and was captured in 346 BC.

Upon his capture, Artaxerxes evidently recognized Mentor's skills, and pardoned him. Immediately, Mentor was sent to aid in the invasion of his former refuge, Egypt. After the defeat of Egypt, Artaxerxes appointed Mentor his commander in the west in 342 BC. One of his actions during his short tenure at this post was to pardon Artabazus, who he allowed to return home, along with Barsine and Memnon. Mentor died after just four years in his post. His daughter later married Nearchus, and Barsine remarried, to Memnon. Memnon received Mentor's command after his brother's death.