Ascendancy of the Red Douglas
Following the acts of attainder, Angus received the vast fiefdom and superiority of the Lordship of Douglas, obtaining a charter of confirmation in 1457
In 1460, Angus accompanied King James to the siege of Roxburgh Castle, held by the English. The castle, which had been a thorn in the side of successive Scottish monarchs, contained a garrison loyal to the House of York. James had decided to back the House of Lancaster. On 3 August James' queen, Mary of Gueldres, had arrived to inspire enthusiasm amongst the besiegers. Overseeing the cannonade to greet his wife, James stood too close to one of the bombards. When it exploded, it killed him, and wounded Angus, his constant companion. Though wounded, Angus was still able to take command of much of the siege, and the castle was stormed and taken within the week. On 10 August, Angus attended the coronation of James III at Kelso Abbey, where he placed the Crown upon the nine-year-old King's head, as was his right as Lord Abernethy. He is supposed to have proclaimed: "There! Now that I have set it upon your Grace's head, let me see who will be so bold as to move it."
Angus had already become enormously powerful during the reign of James II and during the Regency for his son; his power continued to grow. Following the forfeiture of the Black Douglases, the Scots parliament had decreed that no Douglas should have a hereditary right to the wardenship of the Marches, Angus continued to exercise command over the East and Middle Marches, and was appointed lieutenant of the Realm by the Queen-regent Mary of Gueldres.
In the following year, Angus spent a great deal of time consolidating his estates, placing trusted vassals in charge of the more far-flung properties. In 1462, he received all of the goods, lands and rents of the adherents of the forfeited Douglases in Roxburghshire, apart from those already owned by his brother William Douglas of Cluny.
In the same year, Angus was involved in negotiations with the Lancastrian Henry VI of England. Henry had obtained a bond from Angus to aid him with "a stipulated force for the recovery of that realm [England] from Henry's rebels and enemies". For Henry's part, he had promised Angus, on the recovery of his kingdom, "to make the saide erle sufficiently and suerly after the lawes of England a duke withynne the said reavme of England, with stile, astate, honure and name of a duke". The dukedom brought with it a castle and land to the value of 2000 marks.
Angus was unable to pursue his plans for an English dukedom: He died on 12 March 1463 at Abernethy.