Later, after Gandalf saves them (with the help of Bilbo's quick banter as an added distraction), Thorin discovered the legendary sword Orcrist in the troll-hoard along with Glamdring, which Gandalf took for himself. On the journey the company encountered a band of trolls, and Thorin alone of the Dwarves was not taken unawares, though he was forced to surrender when the trolls threatened kill Bilbo. The dwarves, Bilbo, and Gandalf experienced a number of dangers on their journey to the Lonely Mountain. Thorin's intent was to infiltrate the Lonely Mountain and steal back the treasure from Smaug. After receiving a map his father had drawn for him and following advice from Gandalf the Grey, he took a contingent of Twelve Dwarves to visit Bilbo Baggins in order to hire him as a burglar for their cause.

Thorin became determined to reclaim his homeland and destroy the beast that brought such misery upon his people. In TA 2941, Thorin had become the respected leader of Durin's Folk in the new settlement in Ered Luin. For nearly a century, Thorin II tended to his people, building then numbers and wealth.

Thorin eventually inherited his father's short broadsword, Deathless, and thus became King-in-Exile of Durin's Folk after his father went missing, and presumably died in the dungeons of Dol Guldur. At some point in the fighting, his shield broke, and using an oaken tree branch found on the floor as a shield, he gained the epithet 'Oakenshield,' which would remain with him even in death. Thorin sliced off Azog's arm in retaliation of Azog's murder and desecration of his grandfather Thrór, which was enough to gain Dain Ironfoot the advantage in his skirmish with the pale orc, beheading Azog and avenging Durin's Folk on the Orcs of Moria. This was demonstrated when at the still young age of 53 he participated in the Battle of Azanulbizar in TA 2799, when Thorin marched with one of the Dwarven armies beneath the East-gate of Moria. they lived as exiled in Dunland until 2790, during which time he quickly grew to become a capable, strong, fearless warrior. Thorin developed a long standing grudge with King Thranduil for not helping his grandfather save Erebor from Smaug and for not offering any aid to the refugees afterward. When no one came to the aid of the surviving Dwarves, his once proud and noble people were forced into exile.

As a young Dwarf Prince, Thorin witnessed the destruction and terror wrought when the great fire-breathing dragon Smaug attacked the Dwarf Kingdom of the Lonely Mountain in TA 2770. He was the eldest grandson of the Dwarf- king Thrór. 2746 to Dwarven prince Thráin II and Frís in the mountain city of The Lonely Mountain. Master of the golden harp and a tenacious fighter, Thorin II was born in T.A. Thorin was best known for his deeds as leader of a company that infiltrated the lost Kingdom under the Mountain to take it back from Smaug and for leading an alliance of Men, Dwarves, and Elves in the Battle of the Five Armies. Thorin II, also called the Oakenshield, King under the Mountain or the Mountain King, was the son of Thráin II, the older brother of Frerin and Dís, the grandson of King Thrór and the uncle of Fíli and Kíli. ' I am Thorin son of Thrain son of Thror King under the Mountain! I return!' ― The Hobbit, A Warm Welcome

Thror the last king under the mountain