Early in 1642, Dorset joined the king at York, and pledged himself to support a troop of sixty horse. He attested to the king's declaration on 15 June 1642, that he abhorred the idea of war. In July he attended the queen in Holland, but returned before the king's standard was raised at Nottingham. On 25 August he was sent, with Lord Southampton and Sir John Colepeper, to treat with the parliamentary leaders. On the same date, parliamentary soldiers plundered Knole House. He was present at the Battle of Edgehill. He may have been in charge of the young princes as James II wrote in 1679 that "the old Earl of Dorset at Edgehill, being commanded by the king, my father, to go and carry the prince and myself up the hill out of the battle, refused to do it, and said he would not be thought a coward for ever a king's son in Christendom". He went to Oxford with the king, and more than once protested against the continuation of the war. He made a speech at the council table against one by the Earl of Bristol and this was circulated as a tract on 18 January 1643. He was made a commissioner of the king's treasury on 7 March 1643, and was Lord Chamberlain of the household from 21 January 1644 to 27 April 1646. Early in 1644, he was also entrusted with the privy seal and the presidency of the council. He made sensible speeches, which were printed in Oxford and London as "shewing his good affection to the Parliament and the whole state of this Kingdom". He signed the letter asking Essex to promote peace, in January 1644. He was one of the committee charged with the defence of Oxford; and was nominated by Charles in December 1645 one of those to whom he would entrust the militia. He was one of the signatories to the capitulation of Oxford on 24 June 1646.
