Angus gave a strong support to Morton during the attack upon the latter, made a vain attempt to rescue him, and was declared guilty of high treason on June 2, 1581. He then entered into correspondence with the English government for an invasion of Scotland to rescue Morton, and on the latter's execution in June went in exile to London, where he was welcomed by Queen Elizabeth.
After the Raid of Ruthven in 1582 Angus returned to Scotland and was reconciled to King James, but soon afterwards the king shook off the control of the Earls of Mar and Gowrie, and Angus was again banished from Court.
In 1584 he joined the rebellion of Mar and Glamis, but the movement failed, and the insurgents fled to Berwick-upon-Tweed. Later they took up residence at Newcastle-upon-Tyne, which became a centre of Presbyterianism and of projects against the Scottish government, encouraged by Elizabeth, who regarded the banished lords as friends of the English and antagonists of the French interest.
In February 1585 they came to London, and cleared themselves of the accusation of plotting against James's life. A plan was then prepared for their restoration and for the overthrow of James Stewart, Earl of Arran. In October they invaded Scotland and gained an easy victory over Arran, captured Stirling Castle with the King in November, and secured from him the restoration of their estates, and the control of the government.
In January 1586 he was granted the Earldom of Morton with the lands entailed upon him by his uncle; this made him 5th Earl of Morton. In 1586 Angus was appointed Warden of the Marches and Lieutenant-General on the Border, and performed good services in restoring order; but he was unable to overcome the king's hostility to the establishment of Presbyterian government.
He died on 4 August 1588 at Smeaton, near East Linton, Haddingtonshire. According to a 1591 tract his death was caused by witchcraft. The Newes from Scotland reports that he came under a disease which his physician found to be so strange there could be no cure or remedy causing him to languish until his death. The disease was later found to be the cause of witchcraft, likely due to poisoning. During the North Berwick witch trials, Guilles Duncan named Barbara Naper as his killer.
The Earl of Angus was married three times: (1) 13 June 1573 Mary Erskine, a daughter of the 1st Earl of Mar; (2) 25 December 1575 (divorced 1587) Margaret, a daughter of George Leslie, 4th Earl of Rothes; (3) 29 July 1587 Jean, a daughter of John Lyon, 8th Lord Glamis, with whom he had a daughter Margaret, who died young.
Angus began proceedings to divorce Margaret Leslie in August 1586, which were strongly opposed by the Earl of Rothes who considered he had been Angus's ally during his banishment. A French diplomat in Edinburgh, Camille de Preau, sieur de Courcelles, heard that Angus claimed she had flirted with a stableboy, which was thought unlikely, and the Earl of Bothwell had joked he would divorce his wife for the same. She was Angus's sister.
He was succeeded in the Earldom of Angus by his cousin William, a descendant of the 5th earl.
A closer line of the Douglas family to the former Earls of Morton inherited the Earldom of Morton.
(For the Morton title, see James Douglas, 4th Earl of Morton and William Douglas, 6th Earl of Morton.)