He was a Yorkshire landowner and politician who sat in the House of Commons at various times between 1625 and 1642. He was executed for his adherence to the Royalist cause during the English Civil War. He was the second but oldest surviving son of Sir Henry Slingsby, who had been High Sheriff of Yorkshire in 1611, and who died in 1634. His uncle was Sir William Slingsby. He was elected Member of Parliament for Knaresborough in the Useless Parliament of 1625. He inherited substantial estates at Scriven and Redhouse in the West Riding on the death of his father in 1634. He In 1638 he was created a baronet. During the Bishops' War, Slingsby served in the Royal army in Scotland. and was chosen to represent Knaresborough again in 1640, after a vigorously-contested election. He sat in both the Short Parliament and Long Parliament, and was a vigorous supporter of the Royalist cause. On the outbreak of civil war, Slingsby offered to raise a regiment for the King, but his offer was declined because of a lack of arms; but he took possession of Knaresborough Castle, forestalling a Parliamentary plan to seize it. In September 1642, he was one of the first wave of Royalist MPs to be deprived of their seats by the Parliamentarian majority, which passed motions declaring them disabled from sitting. In December 1643 he was finally commissioned as a Colonel in the Royalist army, raiding a regiment whose first duty was to escort the Queen from Bridlington after her return from attempting to raise troops in the Netherlands. During 1644 he was besieged in York, but when the city surrendered after the Battle of Marston Moor, Slingsby escaped with his fellow Knaresborough MP Sir Richard Hutton, the younger to rejoin the King, and was present at the decisive defeat at Naseby. He later joined the garrison at Newark, commanded by his brother-in-law Lord Belasyse, which was being besieged by the Scottish army, and which held out until the King joined the Scots and ordered his supporters in Newark to surrender. Slingsby was arrested, possibly only on suspicion, and imprisoned in Hull. Here he attempted to persuade one of the captains of the garrison to agree to deliver the fortification to Royal forces, and gave him a commission signed by the King as Governor of Hull; however, the captain reported the matter to his commanding officer, sealing Slingsby's fate. At first he was simply imprisoned at York, but following a further royal plot against the Commonwealth in 1658 he was brought before the High Court and charged with treason. Convicted on the basis of his attempts to corrupt the garrison while imprisoned in Hull, he was initially sentenced to be hanged, drawn and quartered, but the sentence was later commuted to beheading, and he was executed on Tower Hill on 8 June 1658, together with fellow conspirator the Revd John Hewett.