Mithrenes (Greek: Mιθρένης or Mιθρίνης) was a Persian commander of the force that garrisoned the citadel of Sardis. According to Cyril Toumanoff, he was also a member of the Orontid dynasty, of Iranian origin. After the battle of the Granicus Mithrines surrendered voluntarily to Alexander the Great, and was treated by him with great distinction. He fought for Alexander at Gaugamela, and ironically he was fighting against an army that included his father Orontes II. After the battle, Alexander appointed him Satrap of Armenia, as his father had been.
It's not clear, however, whether Mithrenes actually managed to take control of his satrapy. According to Curtius, in his speech given at Hecatompylos in 330 BC Alexander the Great listed Armenia among lands conquered by Macedonians, implying that Mithrenes succeeded in conquering it; on the other hand, Justin reproduced Pompeius Trogus' rendition of a speech attributed to Mithridates VI of Pontus, which mentioned that Alexander did not conquer Armenia.
In summary, Mithrenes ruled on behalf of the new Macedonian regime. However, after the death of Alexander III, Neoptolemus was made Satrap of Armenia from 323 to 321 BC.
After the death of Neoptolemus, and the struggles going on with the Diadochi it seems Mithrenes not only returned to his ancestral seat but declared himself king.