Grór was born into the House of Durin during its last troubled years in the Grey Mountains. He was the youngest of the three sons of King Dáin I, who ruled Durin's folk during their wars with the dragons of the north. When Grór was just twenty-six years old, a Cold-drake attacked Dáin's halls, and slew the King along with his second son, Frór.
The two remaining brothers resolved to flee the Grey Mountains with their people and settle elsewhere. As the elder brother, Thrór inherited the Kingship, and led his people back to Erebor in the east. Young Grór travelled still farther east, to the Iron Hills, where he founded a realm of his own.
Grór saw many mishaps and tragedies during his reign. The kingdom of his brother Thrór was destroyed by the dragon Smaug, and Grór's son and heir, Náin, was killed at the Battle of Nanduhirion before he could inherit. Grór's rule passed instead to his grandson Dáin II Ironfoot. Many years later, at the Battle of Five Armies, Thrór's last descendant, Thorin Oakenshield, was lost. Dáin inherited the high Kingship of Durin's folk, a title that remained with the descendants of Grór.