Although his father was a Royalist, when the First English Civil War began in August 1642 Montagu followed his Parliamentarian cousin the Earl of Manchester and served under him in the Eastern Association. He raised a regiment of infantry which during the 1644 campaign fought at Marston Moor, the Siege of York and Second Newbury. Despite his family relationship, Montagu supported those in Parliament who expressed dissatisfaction with the conduct of the war by Manchester and the Earl of Essex.