Born in the Halls of Grór amidst the Iron Hills of northern Rhovanion, Thonin and his younger brother Bronin entered Eriador for the first time in the year 2941 of the Third Age. Their father Hofar, son of Hrór, had been sent to the Blue Mountains as a representative of Lord Dáin Ironfoot to a meeting summoned by King-in-Exile Thorin Oakenshield; the latter requested help in reclaiming his fathers' kingdom of Erebor from the dragon Smaug.

Although Oakenshield was eventually denied assistance from the other Dwarf-realms, the brothers chanced upon the departure of Thorin's own company and came back to Rhovanion with an inkling that great change was coming. Indeed, when a raven flew in from the Lonely Mountain bearing news of Smaug's demise, the brothers' intuition proved accurate. After the ensuing Battle of the Five Armies, Dáin succeeded the late Thorin as King under the Mountain, and a new era of prosperity began for the Dwarves of Erebor and the neighbouring Men of Dale.

75 years later, the brothers had become traders and together headed the Ironhelm Company, versed in the recovery of valuable artifacts as well as the manufacturing of armour and weapons; but their business was stalling. As Thonin neared his hundredth birthday, they agreed to travel once again to the Blue Mountains alongside Thorin's former companions Dwalin and Dori, advising them in seeking overdue tribute from the Halls of Thorin as legal and financial experts… and bettering their own fortunes in the process.

While their travels troughout Eriador since their arrival from the Iron Hills in TA 3016 were met with indisputable entrepreneurial success, the Ironhelm brothers' near-death experience in the Barrow-downs forced them to reevaluate their ways of doing business. The younger Bronin settled in Bree-land and became the face of the company, while his former bodyguard Thonin scoured the lands for valuables on his own as a champion-for-hire, eventually joining the Blue Mountains Regiment.