Mithrenes was an Armenian

Armenians

Armenian people or Armenians are a nation and ethnic group native to the Armenian Highland.The largest concentration is in Armenia having a nearly-homogeneous population with 97.9% or 3,145,354 being ethnic Armenian....

 commander of the Persian

Achaemenid Empire

The Achaemenid Empire , sometimes known as First Persian Empire and/or Persian Empire, was founded in the 6th century BCE by Cyrus the Great who overthrew the Median confederation...

 force which garrisoned the citadel of Sardis

Sardis

Sardis or Sardes was an ancient city at the location of modern Sart in Turkey's Manisa Province...

.
After the battle of the Granicus

Battle of the Granicus

The Battle of the Granicus River in May 334 BC was the first of three major battles fought between Alexander the Great and the Persian Empire...

 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

Orontes II

Orontes II was the son of Orontes I.After Codomannus ascended the throne of Persia as Darius III in 336 BC Orontes was given the Satrapy of Armenia to rule....

.
After the battle, Alexander appointed him Satrap of Armenia

Orontid Dynasty

The Orontid Dynasty The Orontid Dynasty The Orontid Dynasty (also known by their native name, Yervanduni was a hereditary Armenian dynasty and the rulers of the successor state to the Iron Age kingdom of Ararat...

, as his father had been.

It's not clear, however, whether Mithrenes actually managed to take control of his satrapy. According to Curtius

Quintus Curtius Rufus

Quintus Curtius Rufus was a Roman historian, writing probably during the reign of the Emperor Claudius or Vespasian. His only surviving work, Historiae Alexandri Magni, is a biography of Alexander the Great in Latin in ten books, of which the first two are lost, and the remaining eight are...

, in his speech given at Hecatompylos

Hecatompylos

Hecatompylos was an ancient city in west Khurasan, Iran, which was the capital of the Parthian Arsacid dynasty by 200 BCE. The Greek name Hekatompylos means "one hundred gates", but this title was commonly used for cities which had more than the traditional four gates...

 in 330 BC

330 BC

Year 330 BC was a year of the pre-Julian Roman calendar. At the time, it was known as the Year of the Consulship of Crassus and Venno...

 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'

Gnaeus Pompeius Trogus

Gnaeus Pompēius Trōgus, known as Pompeius Trogus, Pompey Trogue, or Trogue Pompey, was a 1st century BC Roman historian of the Celtic tribe of the Vocontii in Gallia Narbonensis, flourished during the age of Augustus, nearly contemporary with Livy.His grandfather served in the war against Sertorius...

 rendition of a speech attributed to Mithridates VI of Pontus

Mithridates VI of Pontus

Mithridates VI or Mithradates VI Mithradates , from Old Persian Mithradatha, "gift of Mithra"; 134 BC – 63 BC, also known as Mithradates the Great and Eupator Dionysius, was king of Pontus and Armenia Minor in northern Anatolia from about 120 BC to 63 BC...

, which mentioned that Alexander didn't 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 - 321 BC.

After the death of Neoptolemus, and the struggles going on with the Diadochi

Diadochi

The Diadochi were the rival generals, family and friends of Alexander the Great who fought for the control of Alexander's empire after his death in 323 BC...

it seems Mithrenes not only returned to his ancestral seat but declared himself king.