Marchmont House lies on the east side of the small town of Greenlaw, and near the former village of Polwarth in Berwickshire, in the Scottish Borders area of Scotland. It is about five miles (8 km) south west of Duns, about 19 miles (31 km) west of Berwick-upon-Tweed and about 40 miles (64 km) south east of Edinburgh. Situated in a gently undulating landscape, the estate is intersected by Blackadder Water, and its tributary burns. With the Lammermuir Hills to the north and views towards the Cheviot Hills in the south, this part of Berwickshire, sometimes referred to as the Merse, is very scenic and contains rich and fertile agricultural land.
The Palladian house was built by Hugh Hume-Campbell, 3rd Earl of Marchmont, in 1750. His bust, albeit a plaster copy, sits on a wall bracket in the saloon. The original is in Mellerstain House, another important Borders house which has intimate historical links with Marchmont. Before its completion the family lived in Redbraes Castle, the diminutive ruins of which can be seen in front of the present house by the steading. At Redbraes lived Sir Patrick Hume of Polwarth, later Patrick Hume, 1st Earl of Marchmont. His portrait by William Aikman, c 1720, and framed by a George II period gilt mirror, hangs above the chimneypiece in the drawing room. It is not coincidence that a near identical portrait hangs in the Music Room at Mellerstain.
Having been implicated in the Rye House Plot, Sir Patrick was forced into hiding in the vaults of Polwarth Church, close to the house. His daughter, Lady Grisell Hume, (later Grizel Baillie) smuggled food to him, and her well trod path from Redbraes to the church became known as ‘Lady’s Walk’. Lady Grisell’s life was an epic story of selfless bravery, for which she eventually found reward in a long and happy life. She wrote her memoirs which were handed down to her daughter, who in turn transcribed them for future generations. In them she tells of the fearful chapters she endured as a young teenager aiding her father and family. Sir Patrick fled to Holland in 1684, again assisted by his daughter, but made a glorious return with King William who reinstated their lands and created Sir Patrick the first Earl of Marchmont in 1697. He was later made Lord Chancellor, and then in 1698 he was elevated to "the highest official position in the kingdom, that of the King's High Commissioner to the Parliament" The king's gratitude was symbolised by his granting Sir Patrick permission to place an orange, bearing the Imperial crown, in his coat of arms. An example of this can be seen clearly on the carved coat of arms on the east gable of Polwarth church restored by, and once a self-imposed prison of, Sir Patrick.