Following the Glorious Revolution, he left Mountjoy's regiment, which had remained loyal to James II, and joined a Protestant militia in Derry shortly before the siege of the city began in April 1689.
[1] During the Siege of Derry, Mitchelburne was appointed colonel by the joint governors, George Walker and Henry Baker, and became the most senior Williamite army officer in the city.
He supervised the construction of new fortifications, and in July erected a gallows on the battlements, threatening to hang prisoners in response to the Jacobite tactic of driving Protestant refugees up to the city walls.
He was directed to capture of Sligo from the Jacobites in July 1691, but command of the attacking Williamite force was soon passed on to Arthur Forbes, 1st Earl of Granard.
[2] He was briefly made governor of Sligo after Granard secured the town, but within a month was recalled to Dublin to face charges of corruption, though these were later dismissed.