At the time of Alexander's invasion of Asia Minor the district of Pontos, on the southern shores of the Black Sea, was a satrapy of the Persian Empire. Subsequently it became a quasi-independent territory under the rule of the Mithradatic dynasty who claimed descent from Darius I of Persia.
Mithradates was the son of the Iranian nobleman Mithridates satrap of Cius. He and his father served with Antigonos, King of Asia Minor and Syria. Antigonos became suspicious of their loyalty. The father was killed and Mithradates fled the military camp of the king. After the defeat and the death of Antigonos in the battle of Ipsos 301 BC, Mithradates managed to occupy the fortress of Cimiata (Κιμιάτα) at the Olgassys Mountains in Paphlagonia. In the following twenty years he extended his rule over other cities of Pontus including Amasia which became the first capital of the kingdom. After repelling successfully Seleucus, he felt sufficiently strong to assume the title of King (Βασιλεύς) 281 BC. The city of Amastris (Αμαστρις) was given to his son Ariobarzanes by the dynast Eumenes 279 BC. Mithradates formed alliances with Byzantium, Chalcedon and Heraclea Pontica. He acquired support from the Gauls (who later settled in Asia Minor) in order to overthrow a force sent against him by Ptolemy, king of Egypt.

Mithradates I issued no coins in his name. He was succeeded by his son Ariobarzanes (Αριοβαρζάνης) 265-250 BC.

References

Sear David R.   1979
Greek Coins and Their Values, Vol. 2 Asia and Africa
Sear Publications, 1979, p. 679
Wikipedia   2017
https://de.wikipedia.org/wiki/Mithridates_I._(Pontos)
 
Χριστόπουλος Γεώργιος (Διευθυντής Εκδόσεως)  1973
Ιστορία του Ελληνικού Εθνους
Τόμος Δ', Εκδοτική Αθηνών, 1973, Σελ. 400