A Greek nobleman, possibly from Miletus in Asia Minor, Timarchus was a friend of the Seleucid prince Antiochus IV Epiphanes during his time as a hostage to the Roman Republic.

He was appointed satrap of Media in western Iran when Antiochus IV Epiphanes became king in 175 BC, and his brother Heracleides became minister of the royal finances.

On his coins, Timarchus introduced the epithet "Great King" (Basileus Megas) which was the traditional Achaemenid title and may reflect an effort to gather support from the natives in a time when the Seleucid empire lost ground in Iran.

Timarchus was survived and avenged by his brother Heracleides, who eventually became champion of Alexander Balas, a boy who he claimed was a natural son of Antiochus IV.

Heracleides convinced the Roman Senate to support the young pretender against Demetrius, who was defeated and killed in 150 BC.