By his marriage with the Lady Lucy, daughter of Robert Rich, Earl of Warwick, he became allied to the leaders of the opposition among the peers, and during the Long Parliament his vote was almost always given with the popular party among the Lords. He was appointed Lord Lieutenant of Cornwall in 1642, and became colonel of a regiment of foot in the army of Lord Essex. He fought at Edgehill, and commanded a brigade in the battle of Newbury. In 1644 he held the position of Field Marshal in the army of Essex, and he was with him in the west when he advanced further into Cornwall, getting into a position which eventually led to a humiliating defeat, among a population fanatically affected to the Royal side.

The King with Prince Maurice was in full pursuit, driving him into a corner, the narrow extremity of Cornwall. The King was now joined by Sir Richard Grenville with Cornish levies, cutting off some of the Parliamentarian foraging parties. But the sea as yet was open, and the Earl of Warwick, who attended the motions of the army, was off the coast. "It was therefore now resolved to make Essex's quarters yet straiter, and to cut off even his provisions by sea, or a good part thereof." Fowey was in the possession of Essex, "and it was exceedingly wondered at by all men, that he being so long possessed of Foy, did not put strong guards into the place, by which he might have prevented his army's being brought into these extreme necessities." Sir Richard Grenville possessed himself of Lanbetherick, a strong house belonging to Lord Roberts, and lying between the Parliamentarian army and the little harbour, and Sir Jacob Astley made himself master of View Hall, which was opposite to Fowey, and then cut off supplies from reaching the camp of Essex.

For eight or ten days the armies lay inactive, and then Charles drew closer the toils in which the hostile army was held. He drove them from a rising ground called Beacon Hill, and immediately raised a square work on it, and placed there a battery that threw a plunging fire into their quarters. Then Goring was sent with the greatest part of the royal horse, and fifteen hundred foot, to S. Blazey, to drive the enemy yet closer together, and to cut off the provisions they received from that direction. The dashing, daring Goring executed his commission with complete success, and the Parliamentarians were reduced to that small strip of land that lay between the river Fowey and that of S. Blazey, which was not above two miles in breadth and little more in width, and which had already been eaten bare by the cavalry. The destruction of the whole army now appeared inevitable; but through the carelessness of Goring, one dark night all the horse of the enemy were allowed to slip unperceived through the lines, on the night of the 30th and 31st August, 1644, and the Earl of Essex with Lord Roberts and many of his officers fought his way to the shore near the mouth of the Fowey, and there they embarked on board a ship which Warwick had sent round, and sailed away to Plymouth on September 1st, leaving the foot, cannon, and ammunition to the care of the gallant Skippon, who had nothing left for it but to make the best capitulation he could. Essex left Roberts in command at Plymouth, which he successfully defended against the attacks made during the ensuing months, and his popularity is attested by the petitions made by the Plymothians that he might be left in command of the town.

Lord Roberts must have suffered considerably in the Civil War, for his estates were alienated from him and granted by the King to Sir Richard Grenville, whilst his home of Lanhydrock was occupied by the Royalists, and his children were detained from him. He was a staunch Presbyterian, hating Prelacy, believing in exclusive salvation, the perquisite of those who believed in Calvin, and he had no love for the Independents.

Since 1643 an assembly for the regulation of religion had been sitting at Westminster. It had substituted Presbyterianism for the Episcopacy, as the established religion of England, and had abolished the Prayer Book and issued in its stead a book called the Directory. These changes had been confirmed by Parliament. But this settlement of the religious question was quite contrary to the views of the Army, which was mainly—at all events that portion commanded by Cromwell in the North—composed of Independents.

Sir Harry Vane, Oliver Cromwell, Nathaniel Fiennes, and Oliver St. John, the solicitor-general, were regarded as their leaders. Dissentions broke out in the House of Commons; Cromwell and Lord Manchester cast imputations on each other. Cromwell desired to remodel the Army, of which the House of Commons had already become suspicious, and how to effect this project was the difficulty, and his object could only be attained by a circuitous course. At his instance a committee was chosen to frame what was called the "Self-denying Ordinance," by which the members of both Houses were excluded from all civil and military employments, except a few offices which were specified. After a great debate, the Ordinance passed both Houses (April 3rd, 1645), and Essex, Warwick, Manchester, Denbigh, Roberts, and many others resigned their commands, and received the thanks of Parliament for their past services.

Lord Roberts's zeal for the cause rapidly cooled. He and Essex both protested against the passing of the Ordinance on March 13th, 1646, that made the new Presbyterian Established "Church" subordinate to Parliament.

On January 3rd, 1648, it was passed in both Houses that thenceforth no addresses should any more be sent to the King; he was virtually dethroned, and the whole constitution was formally overthrown; and by orders from the Army the King was shut up in close confinement, his servants removed, and his correspondence with his friends prevented. When the Army threatened to intervene, Roberts deemed it his most prudent course to absent himself from the House of Lords, and suffer the act to pass.

He took no part in the trial of the King, and after the execution of Charles withdrew from further share in public affairs. He was, however, not hostile to the Protectorate, and at the Installation of the Protector he suffered his son to be in his train.

At the Restoration he was received into favour, and became a Privy Councillor, and was appointed Lord Deputy of Ireland, and Lord Keeper of the Privy Seal in May, 1661. In July, 1679, he was created Viscount Bodmin and Earl of Radnor.