She chose to escape on 2 May 1568 from Lochleven with the aid of Sir William's brother George and a young orphaned cousin named William Douglas who also lived at the castle and may or may not have been the earl's illegitimate son. When Sir William learned of his royal captive's escape, he was so distressed that he attempted to stab himself with his own dagger. In October 1570 he was the keeper of the Earl of Northumberland at Lochleven castle and wrote to the English diplomat Thomas Randolph mentioning that Loch Leven was liable to freeze. The title Earl of Morton was declared forfeit in 1581 when Regent Morton, the 4th earl, was attainted; and the title was granted to John Maxwell, 7th Lord Maxwell, a grandson of the 3rd earl. While Regent Morton was on trial in January 1581, William and other leading members of the family were not allowed to come to Edinburgh, and in March he was ordered to live north of Cromarty. A year later he joined in the Raid of Ruthven, and when this faction was defeated he was exiled in France at La Rochelle, returning in 1586. The 17th century historian David Hume of Godscroft relates that Agnes Leslie wrote to her husband saying she would prevent their son Robert from joining him at the Lords Enterprisers attempt to take Stirling Castle in 1584, saying it was a foolish work that would ruin them. William replied that their course was honourable, and intended for the good of the church, and he trusted in providence. Robert and their son-in-law Laurence Oliphant were banished to France despite their mother's efforts, and were lost at sea in a battle with "Hollanders" or pirates. In 1586, the attainder on the Morton earldom was reversed and the title returned to the 4th earl's family. By the 4th earl's will, on the death of Archibald Douglas, 8th Earl of Angus in 1588, William Douglas succeeded to the earldom of Morton, which brought him additional lands and houses including Dalkeith Palace, Aberdour Castle, Auchterhouse and Drochil Castle. In May 1590 he hosted the Danish Admiral Peder Munk at the Newhouse of Lochleven. Munk had been at Falkland Palace to accept the property as part of the dowry of Anne of Denmark. William wrote a short history of the Scottish reformation and reigns of Mary and James VI briefly mentioning the Siege of Leith, the Battle of Carberry Hill, the murder of David Rizzio, and the Ruthven Raid.