Grór was born into the House of Durin in 2763 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 war with the Dragons of the north. When Grór was just twenty-six years old, a Cold-drake attacked Dáin's halls, and slew the father along with his brother, Frór.
The two remaining brothers resolved to leave 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 traveled still farther east, to the Iron Hills, where he founded a realm of his own becoming the first Lord of the Iron Hills.
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. But during this troubled time Grór's realm became the strongest in the North. Being the only realm standing between Sauron and his plans to destroy Rivendell and taking back the lands of Angmar. Also, many of Durin's folk fleeing from Smaug and those wandering in exile came to the Iron Hills, bolstering their numbers. 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.