The gatehouse to Thurso Castle.

The first fortification at Thurso was raised by the Norwegians probably in the form of an earth and timber structure around 1157. The Norwegians dominated the north and west coasts of Scotland until the mid-13th century and Thurso, which was centrally located on the route from Norway to the Western Isles, was chosen as a key residence to serve the Earls of Orkney and Caithness.

By the 17th century vast swathes of Caithness and Sutherland were under the control of the powerful Sinclair family. Around 1660 George, 6th Earl of Caithness started work on a new castle, overseen by Donald Ross and became his primary residence. He was an important magnate in northern Scotland but found himself embroiled in a bitter land dispute with the Campbells of Glenorchy. Attempts to resolve the issue through the courts were followed by a decision by the King in favour of the Campbells. This prompted George Sinclair to mount a guerrilla campaign against his rivals.

Thurso castle, along with other Sinclair properties were seized by the Campbells but then attacked and badly damaged by George's men. The dispute was eventually resolved following intervention by James, Duke of York (later James VII and II) and thereafter Thurso was repaired.

The castle was substantially rebuilt in 1872 by Sir Tollemache Sinclair. The new structure took the form of a Scottish Baronial Gothic mansion. Large swathes of the 17th century castle were either demolished or incorporated into the new structure. The build quality of the new mansion was poor however and in 1952 much of the structure was demolished for safety reasons leaving just the sections visible today.