Returned for Northumberland as a Whig with the support of Lord Carlisle at a by-election in 1716, Delaval voted with the Government in all recorded divisions. Returning to sea in March 1719, he wrote to his father:
The Admiralty have ... given me a commission to command the Gosport, a new forty gun ship. She is at Deptford and I shall go down on Saturday to put her in commission.
In April he was sent to intercept the Jacobite expedition and in June he was ordered to the Baltic.2HMC 13th Rep. VI, 193-6. He did not stand in 1722, though he appears to have left the navy and to have been back in England at the time. In 1723 his uncle, Admiral George Delaval, who wrote of him ‘I don’t know any young gentleman so well liked and so generally esteemed’, left him Seaton Delaval, the construction of which by Vanbrugh had recently begun. Alarmed at the outlay involved, Delaval wrote to his father, 6 June 1724:
All the scheme I have about the house at present, is to get it covered, for, as we go on, the expense is by far too great, and will make me very inconvenient.
By 1729 ‘this princely mansion, more like a royal palace than the country seat of a subject’, was completed according to Vanbrugh’s original plan at an estimated cost of £10,000.3F. Askham, The Gay Delavals, 23; New Hist. Northumb. ix. 179-80; Beauties of England and Wales (1813), xii. 91.
Delaval died as a result of a fall on the steps of Seaton Delaval on 9 Dec. 1752.