He was present at the Battle of Thermopylae, and after the Persian defeat at the Battle of Salamis, he attempted to convince Xerxes to stay and fight yet another campaign. This time Mardonius could not persuade Xerxes, but when Xerxes left he did become governor of those parts of Greece that had been conquered by the Persians. He subdued Macedon, ruled at that time by King Alexander I, but Alexander himself gave valuable information about Mardonius' plans to the Athenians, saying that, as a Greek, he could not bear to see Greece defeated.