Morton Castle, stands in a very scenic location on a promontory above Morton Loch, 4 miles North of Thornhill and 1½ miles SouthEast of highway A702. The castle is a hall-house, with some additions, that date back to the late 14th century. There would have been kitchens and storage space on the ground floor with a hall above and private rooms in a round corner tower. It was built by the Earls of March on the site of an earlier castle that was destroyed in 1375. In 1459 it was acquired by the Douglases, later made Earls of Morton. It remained occupied until about 1715.

In the 12th century, the honour of Morton was a possession of Dunegal, Lord of Strathnith (Nithsdale). During the reign of Robert the Bruce the lands of Morton were held by Thomas Randolph, later the first Earl of Moray. By 1307, and possibly as early as the 1260s, a castle had been constructed here, on a high defensible promontory surrounded by marshland. Randolph also constructed an enclosed deer park nearby.

The Treaty of Berwick in 1357, which secured the release of David II, also required the Scots to destroy thirteen castles in Nithsdale, including Morton. It is not clear how much, if any, of the original castle remains. The lands of Morton passed to the Earls of March, who probably built (or rebuilt) the existing castle in the early 15th century.

In the mid 15th century the lands were given by James II of Scotland to James Douglas of Dalkeith, later Earl of Morton (although the earldom is named for another Morton in Lothian).

The 4th Earl of Morton was executed in 1580 for his alleged part in the murder of Lord Darnley, and Morton Castle, together with the earldom, briefly passed to John Maxwell, 7th Lord Maxwell, and grandson of the third earl. However, in 1588, James VI led an expedition against the Catholic Maxwells. Morton Castle was taken and burned, and returned to the Earls of Morton, the fourth earl's attainder having been reversed.

In 1608 Morton Castle was sold to William Douglas of Coshogle, who sold it in turn to Drumlanrig, made the first Earl of Queensberry ten years later. Following its abandonment in the 18th century, much stone was carried off until the 1890s when some repairs were carried out. It is now the property of the Duke of Buccleuch, and is cared for by Historic Scotland.