The estate passed to George Baillie's grandson, George Hamilton, second son of the Earl of Haddington, who changed his name to Baillie when he inherited Mellerstain. He commissioned William Adam's famous son, Robert, to complete the building of Mellerstain, which he did between 1770 and 1778.

Surviving drawings suggest that Robert Adam intended to modify the wings designed by his father, but this work was never carried out. The exterior was slightly altered at a later date, when the main entrance was rebuilt in the style of the William Adam wings.

The interior boasts some of Robert Adam's finest work. The library is particularly outstanding, and the plaster ceiling in this room is one of his masterpieces. The interior also includes marble busts by Roubiliac, one of which represents Lady Grisel Baillie.