At the Battle of Marston Moor Lucas swept Fairfax's Yorkshire horse before him, but later in the day he was taken prisoner, in a battle won decisively by Parliament. Exchanged for Parliamentary prisoners during the winter, he defended Berkeley Castle, in Gloucestershire, for a short time against Thomas Rainsborough, but was soon back in the field. As lieutenant-general of all the horse, he accompanied Lord Astley in the last campaign of the first war, and taken prisoner again at Stow-on-the-Wold, he agreed not to bear arms against Parliament in the future.