Mithridates I ruled Parthia from about 171 BC to 132 BC. He inherited a kingdom founded by earlier Arsacid kings and pushed it into a new age of expansion.
At that time, the Seleucid Empire was weakened by internal conflicts. Mithridates took advantage of this and advanced west, capturing Media and then Babylonia, both rich and strategic regions.
In 141 BC, his forces captured Seleucia on the Tigris, one of the most important cities in the East. This gave Parthia control over major trade and tax routes, bringing more wealth and manpower.
Mithridates did not rely only on battlefield victories. He also secured rule by keeping local elites and administration working under Parthian authority, which made newly conquered lands more stable.
Later tradition remembers him as a ruler who turned Parthia into an empire-level power, and many historians call him a 'second founder' of the Arsacid state. During and after his reign, Arsacid kings increasingly used the title 'King of Kings' to express their imperial status.