He returned to England in 1642, and served for a little while in the parliamentary army, where he commanded his ailing father's troop of horse. In April 1643, after his father's death, he deserted to the king at Oxford. Now Earl of Peterborough, he joined the Cavaliers and fought at the battles of Bristol, Gloucester and Newbury in 1643. At Newbury he was wounded in the arm and thigh, and had his horse shot under him. In command of a regiment raised at his own expense he served in the west during the following summer and winter, at Cropredy Bridge and Lostwithiel in 1644. In about December 1644, he married Lady Penelope O'Brien (the only daughter of the 5th Earl of Thomond) and they had two daughters. He was in France during the later phases of the struggle. In 1646 he returned to England and compounded for his estates. A private interview with Charles I as he passed through Ampthill to Hampton Court, in the summer of 1647, prompted him to make a last effort on the king's behalf, and in July 1648 he united with George Villiers, 2nd Duke of Buckingham and Henry Rich, 1st Earl of Holland in raising the royal standard at Dorking. The plan was to seize Reigate, but foiled in this, the insurgents were driven back on Kingston, and eventually dispersed in the neighbourhood of Harrow by the parliamentary forces (7 July). Mordaunt was severely wounded, but escaped to Antwerp, and in the following year returned to England and recompounded for his estates (May 1649).