Although Duns Castle appears to be a 19th century castellated mansion, taking its current Gothic form from the work done by James Gillespie Graham between 1818 and 1822, it also contains much earlier work.

Originally Duns Castle was a large stone keep, built between 1316 and 1320, measuring around 15 metres by 10.5 metres. Probably in the 15th or 16th centuries a square wing was added to the keep, the new addition measuring around 9 metres square with walls some 2.4 metres thick.

Further additions were made in 1698, from 1774 to 1777, from 1791 to 1798, and then finally between 1818 and 1822.

Duns Castle is owned and lived in by the Hay family, although it is also used for weddings and corporate events. The Pavilion Lodge, an 18th century gatehouse to the east of the castle, can be rented out as self-catering accommodation.