He took refuge in Ephesus under Persian protection; whence, however, after the battle of the Granicus, fearing the approach of Alexander, he escaped with the Greek mercenaries who garrisoned the place, and fled to the court of Darius.
At the battle of Issus we hear again of Amyntas as a commander of Greek mercenaries in the Persian service and Plutarch and Arrian mention his advice vainly given to Darius shortly before, to await Alexander's approach in the large open plains to the westward of Cilicia.
On the defeat of the Persians at the battle of Issus, Amyntas fled with a large body of Greeks to Tripolis (region of Phoenicia) along with Aristomedes of Pherae and others.
The gates of Pelusium were opened to him on his pretending that he came with authority from Darius : thence he pressed on to Memphis, and being-joined by a large number of Egyptians, defeated in a battle the Persian garrison under Mazaces.
But this victory made his troops overconfident and incautious, and, while they were dispersed for plunder, Mazaces sallied forth upon them, and Amyntas himself was killed with the greater part of his men.