Thorin III Stonehelm was the son and heir of Dáin II Ironfoot of Durin's folk, who was lord of the Dwarves of Erebor and the Iron Hills in Wilderland.

He became King under the Mountain when his father Dáin was killed during the War of the Ring in 3019 T.A.. Thorin III helped rebuild Erebor and Dale, and prospered. His realm became a close ally of the Reunited Kingdom of Arnor and Gondor of King Elessar.

During his rule Gimli, son of Glóin, led a number of Dwarves south to Aglarond, where a new lordship was established, most likely a colony of Durin's Folk under the vassalage of the King of Erebor.

Thorin III had a descendant, Durin VII the Last, who was held to be the final reincarnation of Durin the Deathless. It is unclear whether Durin VII was a son of Thorin III or a later descendant.