The Slingsbys acquired Scriven, a mile from Knaresborough, by marriage in 1333. The MP’s father, a duchy of Lancaster official, made a prestigious match with a sister of the 8th earl of Northumberland (Sir Henry Percy†), and inaugurated the family’s long record of parliamentary service with his return for Knaresborough in 1572. However, he ran into debt in his later years, and died in the Fleet. Slingsby, a duchy official from 1588, did rather better than his father, becoming an ‘undertaker’ for the plantation of 8,000 acres of Irish land in county Tipperary, and acquiring a second Yorkshire estate at Moor Monkton, between Knaresborough and York, in 1595. His younger brother, Sir William*, sat for Knaresborough in the 1597 Parliament, but Slingsby himself did not seek election until after his father’s death; as borough bailiff, he returned himself in 1601 and 1604, a fact the indentures omitted to mention.
Slingsby was appointed to the Council in the North at the start of the new reign, on the recommendation of lord president Burghley (Thomas Cecil†), but he was unsympathetic when approached by the latter’s successor, lord president Sheffield, for a nomination at Knaresborough in October 1603. Some of the townsmen, he insisted, had already promised their voices to George Clifford, 3rd earl of Cumberland, while the chancellor of the duchy, Sir John Fortescue*, also had the right to a nomination. This plausible tale was accompanied by another letter from the burgesses, who advised Sheffield to clear his nomination with Fortescue, and revealed that the other seat was earmarked for Slingsby. In the event, Sheffield seems to have withdrawn, and, as in 1601, the seats went to Slingsby and his brother.
Never a prominent Member, during the 1604 session Slingsby was named to the committee for the bill to confirm the title of those who farmed ancient forest lands in Worcestershire against proceedings by the assarts patentee Otho Nicholson (2 May), a matter of considerable concern to the tenants of Knaresborough forest. On 1 June, Slingsby was among those named to prepare for a conference with the Lords about a book by Bishop Thornborough of Bristol (a pluralist who was also dean of York), which criticized the Commons’ proceedings over the Union, and he was subsequently named to the committee for a private bill concerning a Lincolnshire drainage project (2 July).
By the time the Parliament was dissolved, Slingsby was in deep trouble with the duchy administration, apparently over financial malpractice. In 1609 he was suspended from his receiverships, a development which quickly bankrupted two of his financial agents, and although he raised a storm of protest, duchy officials remained deaf to his pleas. However, in July 1611 the honour of Knaresborough passed to Prince Henry, who appointed Slingsby to the survey commission for his new estates, while four months later he regained control of the duchy revenues when he took office as sheriff of Yorkshire, the official into whose hands his duchy receipts had been sequestrated. As sheriff, Slingsby proved a singularly incompetent incumbent - the circuit judges, outraged at his failure to attend the York assizes in the summer of 1612, fined him £200 for contempt - but he was able to insinuate himself back into the affairs of the honour of Knaresborough.
By 1614, although still under investigation by the duchy, Slingsby was trying to arrange the maximum personal benefit from the Crown’s plans to enclose 21,000 acres of the Knaresborough manorial commons. At the general election, he faced stiff competition, with rival nominations from the duchy, lord president Sheffield and Lord Henry Clifford*, steward of the honour. Writing from London on 9 Mar., he advised the burgesses:
I understand that it is conceived in the country that matter of enclosure of wastes and enfranchising of copyholders will amongst other things be handled at this Parliament, and I think it is not unknown to my neighbours ... that I do very much affect the enclosing of commons as a matter beneficial to the commonwealth ...
The issue which concerned the copyholders most was the precise division of the manorial spoils, and Slingsby’s involvement in this process allowed him to treat the voters in a rather offhanded manner: ‘if I be chosen my voice must go according to my heart and conscience, and ... if they do not like of this opinion of mine I will be well pleased they choose another’. This missive was read to the burgesses on the eve of the election, and while Slingsby probably faced a challenge at the hustings he was returned along with the diplomat William Beecher, who was apparently Clifford’s nominee. Slingsby’s only appearance in the Parliament’s records was as part of a delegation sent to the king on 29 May to explain the Commons’ decision to suspend business following the aspersions cast upon it by Bishop Neile.
The dispute with the duchy reached its nadir in 1615, when Slingsby was committed to the Fleet, but he was released after surrendering his offices. At the 1620 general election he was not only returned for Knaresborough, but also canvassed the freeholders of Claro wapentake for the shire election, on behalf of Sir Thomas Wentworth*.
At the 1624 election Slingsby saw off a challenge from Sir Arthur Mainwaring*, one of Prince Charles’s servants. Once in the Commons, he was named to the committee for a bill discouraging the transfer of lawsuits from local courts to Westminster (9 Mar.), another to confirm an exchange of lands allowing the duke of Buckingham to acquire the London townhouse of the archbishops of York (19 May) and a third to make the estates of lord treasurer Middlesex (Sir Lionel Cranfield*) subject to the payment of his debts (19 May). Except for nominations to the committees for two private estate bills (7, 24 Apr.), he left no further trace on the records of the session.
Slingsby had his son Henry returned in his stead in 1625, but the parliamentary interest at Knaresborough slipped from his control in the following year, and his son was trounced at a poll in 1628. Despite his advancing years, he was appointed vice-president of the Council in the North by Wentworth in 1629. His will of 10 Dec. 1634 made provisions for his younger son Thomas. He died a week later and was buried in Knaresborough parish church, under a memorial effigy completed in his lifetime.