According to the Iliad, he was the king of the Phylace. He was the first man to die in the Trojan War. When the Greeks landed at Troy, no one would step off of his ship, however. There was a prophecy that the first Greek attacker to set foot upon the shores of Troy would die. Protosilaus could stand waiting no longer, and was the first to leave his ship.
For a time he defied fate, killing several Greek fighters. Eventually, however, he was killed by Hector.