MacLellan's Castle stands on the site of the convent of Greyfriars in the centre of Kirkcudbright. Sir Thomas MacLellan acquired the site nine years after the Reformation Act of 1560. MacLellan was an important man in the local community who held the position of Provost of Kirkcudbright between 1576 and 1597. He used the old convent as a source of stone to construct his new castle.

The building is an L-plan tower-house with the addition of a tower in the south-east corner and two projections in the re-entrant angle. These changes to the standard L-plan tower-house mark a move away from defence and towards greater space and comfort.

Despite MacLellan's importance and wealth it appears that the castle interiors may never have been finished. Following Sir Thomas's death in 1597 his descendants sought their fortune in Ireland, showing no interest in the castle at Kirkcudbright. By 1742 the castle was owned by the MacLellans of Orchardton who stripped the building of its roof and contents, leaving an empty shell.