MP for Thirsk from 1640 to 1642 and a follower of the Royalist cause, he raised six regiments of horse and foot soldiers at his own expense, and took part in the battles of Edgehill and Brentford (both in 1642), Newbury (1643), Selby (1644) and Naseby (1645), as well as the sieges of Reading (1643), Bristol and Newark — being wounded several times. He later became Lieutenant-General of the King's forces in the North of England, and Governor of York and of Newark.
Belasyse is considered one of the first members of the Royalist underground organisation, The Sealed Knot (as is his predecessor as Governor of Newark: Sir Richard Willis). During the Commonwealth, Belasyse was in frequent communication with King Charles II and his supporters in Holland, and after the Restoration was appointed Lord-Lieutenant of the East Riding of Yorkshire (1661–1673) and Governor of Hull, while from 1664 to 1666 he held the posts of Captain-General of the forces in Africa and Governor of Tangier (according to Samuel Pepys, Belasyse accepted the post only for the profit it brought).
Pepys also recorded an anecdote about Belasyse's civil war activities in a diary entry on 4 February1665:
"To my office, and there all the morning. At noon, being invited, I to the Sun behind the Change to dinner to my Lord Bellasses – where a great deal of discourse with him – and some good. Among other at table, he told us a very handsome passage of the King’s sending him his message about holding out the town of Newarke, of which he was then governor for the King. This message he sent in a Slugg-bullet, being writ in Cypher and wrapped up in lead and swallowed. So the messenger came to my Lord and told him he had a message from the King, but it was yet in his belly; so they did give him some physic, and out it came. This was a month before the King’s flying to the Scotts; and therein he told him that at such a day, being the 3 or 6 May, he should hear of his being come to the Scotts, being assured by the King of France that in coming to them, he should be used with all the Liberty, Honour and safety that could be desired. And at the just day he did come to the Scotts.
"He told us another odd passage: how the King, having newly put out Prince Rupert of his Generallshipp upon some miscarriage at Bristol, and Sir Rd. Willis of his governorshipp of Newarke at the entreaty of the gentry of the County, and put in my Lord Bellasses – the great officers of the King’s Army mutinyed, and came in that manner, with swords drawn, into the market-place of the town where the King was – which the King hearing, says, “I must to horse.” And there himself personally, when everybody expected they would have been opposed, the King came and cried to the head of the Mutineers, which was Prince Rupert, “Nephew, I command you to be gone!” So the Prince, in all his fury and discontent, withdrew, and his company scattered – which they say was the greatest piece of mutiny in the world."[1]
In 1667 he was appointed Captain of the Gentlemen-at-Arms. However, as a Catholic, Belasyse later had to resign all his appointments as he was unwilling to take the Oath of Conformity introduced under the Test Act.
He was imprisoned in the Tower of London from 1678 to 1684 after false allegations by Titus Oates that he and others planned to raise a Catholic army.
