Mentor in Harpers Dictionary of Classical Antiquities (1898)
A Rhodian Greek who with his brother Memnon served the Persian Artabazus and later King Nectanabis of Egypt. He aided Tennes, king of Sidon, against Darius Ochus, and, when Tennes went over to the Persians, entered the service of Darius, who made him satrap of the western part of Asia Minor (Arrian, Anab. vii. 419).
Mentor of Rhodes in Wikipedia
Mentor of Rhodes (Μέντωρ ὁ Ῥόδιος) (c. 385 BC – 340 BC) was a Greek mercenary who fought both for and against Artaxerxes III of Persia. He is also known as the first husband of Barsine, who later became mistress to Alexander the Great.
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 Philip 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 pharaoh 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.