William Adam began to build the house for Geoerge Baillie in 1725. In the event he was only to build the two projecting wings that flank the northern entrance forecourt. They are both self contained buildings, nearly square with a courtyard in the middle. The east wing was the family house and the west wing was the stable block. It is a mystery why his central block was never built.
It was George Baillie's grandson, also George, who, when he inherited the estate in 1759 called in Robert Adam to complete the house. He designed a building to bridge his father's wings in the castle style rather than the purely classical building that his father had intended and which was his own preferred style, and of which he was the master. So Mellerstain's appearence is something of an anachronistic throw back to the Borders peel-towers. It has rough hewn stone it has a severe and forbidding appearance. In marked contrast to the undisputed glory of Mellerstain which are Robert Adam's masterpiece the fabulous interiors.
George Baillie was the second son of Charles Lord Binning and heir to the sixth Earl of Haddington through an aunt. After the twelth Earl of Haddington died in 1986, the thriteenth earl decided to make Mellerstain the family seat and close down the family's other great house at Tyninghame in East Lothian. He at the same time converted William Adam's Stable block at Mellerstain to make his family a new home.