George was a natural half-brother of Rostom of Imereti on whose death he succeeded in 1605, but his authority was seriously challenged by the energetic prince of Mingrelia, Levan II Dadiani, whose increasing influence over the western Georgian polities George tried to restrict without any success.

In 1623, Levan, with a combined Mingrelian-Abkhaziann army, inflicted a heavy defeat upon the royal troops.

In his quest for allies, George established close ties with the influential eastern Georgian noble Giorgi Saakadze who employed an Imeretian force in his struggle against King Teimuraz I of Kakheti.

This, however, failed to bring the feudal anarchy in Imereti to an end and the unrest continued.

Later in his reign, George III once again campaigned against Levan Dadiani, but was defeated and taken captive.