Maurice was given his own command early in 1643, when Charles sent him to Gloucestershire after the successes of William Waller (March). This first visit to the west saw Maurice victorious at the battle of Ripple Field (13 April 1643), the first defeat inflicted on Waller, although Maurice with 2,000 men did outnumber Waller with 1,500. Soon after his victory, Maurice was recalled by Charles to help with the relief of Reading, but was soon back in the west, although this time not with his own command. On 19 May he left Oxford with a force commanded by the Marquess of Hertford, with orders to join with the army of Sir Ralph Hopton and return to Oxford with the combined force, which was to form part of a strong field army based at Oxford. The two armies met at Chard on 4 June. Together, they had some 6,300 men, but their command structure was a source of potential weakness. Hertford was not a soldier, while Hopton and Maurice both had claims to command. Luckily, Hertford was willing to be a figurehead, while Maurice was satisfied with the command of the cavalry, leaving Hopton in overall charge. Facing them was Waller, whose main concern was to prevent the Royalists reaching Charles at Oxford.
Maurice and his cavalry played a mixed role in the campaign that followed. At the first encounter between the two forces (Chewton Mendip, 10 June 1643), Maurice was for a short time captured, but it was his leadership which prevented a Royalist disaster. In contrast, at the first real battle (Lansdown, 5 July 1643), the Cavalry performed poorly. Their attacks on a strong Parliamentary position failed, and all but 600 of the 2,000 fled the field, leaving the Cornish infantry to save the day for the Royalists, although there is no suggestion that Maurice was at fault. Indeed, the next few days saw Maurice at his best. In the aftermath of Lansdown, Hopton was badly injured in an explosion, leaving Maurice to command the Royalist retreat to Devizes, pursued by Waller's much larger force. Once in Devizes, it was decided to sent to Oxford for help, and Maurice, with Hertford and the Cavalry left Devizes on 10 July, reaching Oxford the next morning, after an impressive 45 mile overnight ride. In Oxford, they found Charles already aware of the danger. On the previous two days he had sent out reinforcements, including a cavalry brigade under Lord Wilmot. Maurice was able to gain another brigade, and on 12 July was sent back with the reinforcements. The following day the new cavalry inflicted a heavy defeat on Waller (battle of Roundway Down, 13 July 1643). Maurice was present, but does not appear to played a major part in the battle.
After Roundway Down, Maurice was given command of the Cornish army. His first engagement was the capture of Bristol (26 July). While Maurice faced the southern defences of the city, Prince Rupert faced the weaker northern walls. This was reflected in their respective plans, with Maurice wishing to run a prolonged siege, and Rupert determined to risk an assault. It was Rupert who prevailed, and the assualt went in on 26 July. The Cornish troops were repulsed from the strong southern defences, losing several of their best commanders, but Rupert was successful, and the city fell. After taking Bristol, the two armies split, Maurice heading back into the west to reduce the last Parliamentary strongholds. At first things went well, and he captured Exeter on 4 September and Dartmouth on 6 October, but he stalled before Plymouth.
The next year he moved on to attack Lyme, the last chain in a string of fortresses that ran from the Bristol channel across to Lyme bay, and the only one still in Parliamentary hands. He began his siege on 20 April 1644, but ran into problems right from the start. The terrain made it hard for him to bombard the city properly, while his lack of a fleet meant he could not stop the Parliamentary fleet from reinforcing the garrison. Meanwhile, the earl of Essex was advancing with a relief force, and at 2 am on 15 June Maurice was forced to abandon the siege, having wasted eight weeks and 1,000 lives, as well as doing much damage to his reputation. However, Essex helped redeem the situation by marching into Cornwall, where he was eventually cornered at Lostwithel by Charles I in person. Maurice played an important role in the battle of Beacon Hill (21 August 1644), commanding one part of the long front upon which Charles chose to attack.
He was also present at the second battle of Newbury (1644), where he commanded a portion of his western army. This force found itself at the heart of the fighting. It was positioned at the village of Speen, on the western side of the King's position. It was here that the Parliamentary flanking attack, again under Waller, hit hardest, and Maurice's troops were driven back out of the village. For a moment, Charles and Maurice, who were at the head of the reserves, were threatened by the melee, but they were soon rescued, and Charles was able to escape with his army intact.
1645 was a year of disasters. At the start of the year Maurice was based at Shrewsbury, the main training base for new recruits from Wales. However, while he was away at Chester, the city fell to Parliament (22 February 1645). This caused the Royalists great problems, blocking the main route for their recruits, and isolating Chester. He was also present at Naseby (14 June 1645), fighting on the right wing. After the defeat, he was appointed governor of Worcester, being fortified in case Oxford became too dangerous. However, by the time Charles needed the refuge, Prince Rupert was in disgrace after surrendering Bristol (October 1645), and Maurice suspect with him. Maurice stood by his brother, defending him to the king, and travelling with him as he attempted to gain a hearing from Charles. Although Rupert was vindicated, the controversy helped destroyed Charles's last army. Maurice and Rupert remained loyal to the end, besieged in Oxford. They surrendered on 22 June 1646, two days before the rest of the garrison, and on 26 June were banished by Parliament. Maurice remained with Rupert in exile, and joined him in virtual piracy, but was lost at sea in 1652