Marchmont House lies on the east side of the village of Greenlaw, and near to a church in 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 scenic and contains rich agricultural land.

The Palladian house was built by Hugh Hume-Campbell, 3rd Earl of Marchmont, in 1750. His bust, a plaster copy, sits on a wall bracket in the saloon. The original is in Mellerstain House, another important Borders house that has historical links with Marchmont. Before its completion, the family lived in Redbraes Castle, the ruins of which can be seen in front of the 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. A nearly identical portrait hangs in the Music Room at Mellerstain.

Having been implicated in the Rye House Plot, Sir Patrick went 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 a story of selfless bravery. She wrote her memoirs which were handed down to her daughter, who in turn transcribed them. 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 symbolized by his granting Sir Patrick permission to place an orange, bearing the Imperial crown, in his coat of arms. This can be seen on the carved coat of arms on the east gable of Polwarth church restored by, and once a self-imposed prison of, Sir Patrick.

The family's prosperity in the late 17th century, brought about by the reward of loyalty to the Crown, made it possible for grand ideas of Marchmont House to be conceived. Thoughts of creating an impressive new family seat inspired Alexander, the second Earl, to draw up designs for the landscaping. He also commissioned William Adam to draw up plans for a new house, however these were considered to be too expensive. Both the 2nd Earl and Adam died before the house was started. It was the 3rd Earl, succeeding in 1740, who became responsible for the execution of the new building.

It is now understood that it was the lesser known architect, Thomas Gibson who designed the house. Colvin firmly attributes the design to Gibson, as does Macaulay. More recently, French, writing in Country Life, suggested that much of the design may be attributed to James Gibbs, the designer of Houghton Hall, since some of the stylistic features of Marchmont are very much in the manner of Gibbs. Marchmont's internal arrangement and elevations reflect those at Houghton Hall, Norfolk. It had been believed that William Adam may have designed the building. His influence may be present in the architecture as well as the garden design, as well as that of the Palladian revival style of Lord Burlington, a friend of the Earl of Marchmont. The second Earl's sister, Lady Grisell, married a Baillie from Mellerstain, where William Adam worked.

The interior is one of the finest in Scotland. Much of the original George II period plasterwork, executed by Thomas Clayton, remains. Clayton was the best plasterer in Scotland of his time and worked for William Adam and others. He worked at other houses, including The Drum in Edinburgh, the Duke's Apartments at Holyroodhouse, at Hamilton Palace, the Duke's house in Lanarkshire, Blair Atholl, Perthshire and Arniston House.

Clayton's best work can still be seen at Marchmont. The Saloon was decorated between 1753 and 1757 with military trophies on the ceiling and walls and family heraldry above door lintels detailing monograms of the 3rd Earl and his second wife. The Drawing Room celebrates the rich effect that the sun and the phases of the moon have on nature. The central panel is surrounded by baskets of fruit, flowers, and arable crops. This theme is echoed in the contemporary carved marble chimney piece decorated with the sun's rays seeming to emanate from the heat of the fire.

Some alterations to the rear elevation of the house were made in 1834–42 by the architect William Burn, but major changes were made in the early 20th century. The last of the Hume family to live in the house was Sir John Hume Campbell who sold it to Robert Finnie McEwen, father of Captain Sir John Helias Finnie McEwen, who inherited a considerable fortune from his uncles who had built the harbour at Rio de Janeiro. From 1914 to 1917 he commissioned Sir Robert Lorimer to make various alterations: a top floor was added and given dormer windows in a steeper pitched roof; the flanking pavilions were connected to the house internally; by lowering the ground at the front of the house the entrance was relocated on the ground floor with the addition of a porch, and the former entrance, now the central window of the first-floor saloon, made redundant.