The son of Thráin I, born after his father had led many of Durin's Folk away from Khazad-dûm to found a new home at Erebor. Thorin was thus the first of Durin's line to be born after their exile from their ancient mansions.
On the death of Thráin in III 2190 Thorin succeeded him to become the second King under the Mountain, and there he ruled for twenty years. At this time many of his scattered people had begun to settle among the Grey Mountains to the west of Erebor, and in III 2210 Thorin abandoned the Lonely Mountain, taking his young son Glóin to join his people among the rich mountains to the west. After his time there was no King under the Mountain for nearly four hundred years, until the Lonely Mountain was resettled by Thorin's descendant Thrór in III 2590.