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 Kingdom was settled. Also during his rule mining for mithril in Moria slowly began again, although Moria was not yet permanently settled.
Thorin III had an heir, (or possibly a later descendant) who was held to be the reincarnation of Durin the Deathless, and who succeeded him at his death as Durin VII the Last.