Both Lord Willoughby and his father supported the King and raised a regiment of cavalry in Lincolnshire on his behalf. Lord Willoughby commanded the Life Guards at Edgehill, where his father was mortally wounded by a shot through the thigh. Willoughby surrendered to the Parliamentarians in order to attend his father, whom he now succeeded as Earl of Lindsey and Lord Great Chamberlain. He was imprisoned in Warwick Castle, where he wrote a defiant declaration and justification of his loyalty to the King. He was not released until a prisoner exchange in July 1643, whereupon he rejoined the King at Oxford and was appointed a Privy Counsellor in December. As colonel of the King's Life Guards of Foot and subsequently lieutenant-general of the Life Guards "and all the foot", he fought at the First Battle of Newbury, Cropredy Bridge, Lostwithiel, Second Battle of Newbury, and was wounded at Naseby. In addition to his military services for the King, Lindsey frequently acted as a commissioner to treat with Parliament and persistently urged reconciliation. The King additionally honored Lindsey with the post of Gentleman of the Bedchamber from 1643 until 1649, and Steward, Keeper and Ranger of Woodstock in 1644.