The title Earl of Morton was created in the Peerage of Scotland in 1458 for James Douglas of Dalkeith. Along with it, the title Lord Aberdour was granted. This latter title is the courtesy title for the eldest son and heir to the Earl of Morton.
The family seat is Dalmahoy Farms, near Kirknewton, West Lothian.
The Douglases of Dalkeith are descended from Andrew Douglas of Hermiston (or Herdmanston) (d.b. 1277), younger son of Archibald I, Lord of Douglas (fl. c. 1198–1238). He was succeeded by his son William Douglas of Hermiston, a signatory of the Ragman Roll in 1296. William of Hermiston's son, James Douglas of Lothian succeeded his father and produced two sons, Sir William Douglas and Sir John Douglas. Sir William Douglas, known as the Knight of Liddesdale or the Flower of Chivalry obtained the privileges of the barony of Dalkeith, in Midlothian, in 1341, and the barony of Aberdour, in Fife, in 1342. Following his murder at the hands of his godson William Douglas, 1st Earl of Douglas, both baronies passed to his nephew, James Douglas, 1st Lord Dalkeith. James Douglas was confirmed in this position when his title was ratified by the Earl of Douglas prior to 1370. The lands of Dalkeith, and Aberdour, in Fife, were combined as a single barony in 1386, with the principal seat at Dalkeith Castle, and a secondary residence at Aberdour Castle. James was the brother of Nicholas Douglas, 1st Lord of Mains.