Achaemenid general, a nephew of King Darius I and married to Darius' daughter Artazostra. In 492 BC he was sent to succeed the satrap (governor) Artaphernes in Ionia, with a special commission to attack Athens and Eretria. Contrary to the usual Achaemenid policy, he abolished the ruling “tyrants” and restored democracies in Ionia, thereby removing a major source of unrest. He then crossed the Hellespont and invaded Thrace and Macedonia. His fleet was wrecked off Mt. Athos with enormous loss, however, and because of this setback he was deprived of his command.

According to the Greek historian Herodotus, Mardonius was one of those who encouraged King Xerxes I, Darius' successor, to invade Greece. After the Achaemenid defeat at Salamis he persuaded Xerxes to return to Asia and himself stayed behind with a large army. He unsuccessfully attempted to separate Athens from the other Greek allies, and, withdrawing from Attica, he finally was defeated and killed in battle at Plataea in September 479.