The Delaval family had owned the estate since the Norman Conquest. By 1717, when Admiral George Delaval bought it from an impoverished kinsman, the existing mansion was unimpressive. George had made his fortune capturing prize ships in the navy and serving as a British envoy under Queen Anne. He wanted something worthy of his new wealth. In 1718 he asked Sir John Vanbrugh - architect of Blenheim Palace and Castle Howard - to advise on modernising the house. Vanbrugh took one look at the site and recommended demolition. Save only the ancient chapel, he said, which is now the parish church of Our Lady. The Admiral took the advice. Construction proceeded under the clerk of works James Mewburn. Neither Vanbrugh nor the Admiral lived to see the building completed. In 1723, before the new hall was even half built, George Delaval was killed in a fall from his horse. A small obelisk by the road north of the hall marks the spot. Only its pedestal survives now. The hall itself was finished in 1728.

The architecture is English Baroque, rooted in the Palladian style that Inigo Jones had introduced to Britain a century earlier. A central block - the corps de logis - contains the state and principal rooms, flanked by two wings. The wings have a centre projection of three bays crowned by a pediment, with seven bays of sash windows on each side rising above a ground-floor arcade. Between the wings is a cour d'honneur, a great open courtyard 180 feet long and 152 and a half wide. The proportions are vast and disciplined at once, with a deliberate weight to the masonry, a heaviness that catches Northumberland light differently in every season. In the forecourt stands a lead figure of David, with empty sling, poised above the crouching form of Goliath. This is an 18th-century copy, possibly by John Cheere, of a 16th-century Italian marble in the manner of Baccio Bandinelli or a follower of Giambologna.

About half a mile east of the hall stands a stone mausoleum in the 400-acre estate park. Its dome is long gone. Its portico still rests on huge monolithic columns. It is surrounded by a circular ha-ha - a stone-faced ditch that creates a boundary without breaking the view. The mausoleum was erected by Lord Delaval for his only son John, who died in 1775 aged nineteen. The cause of death, as recorded by a family chronicler, was that he had been 'kicked in a vital organ by a laundry maid to whom he was paying his addresses.' No one was ever buried in the mausoleum. It was never consecrated. The unfortunate John Delaval was buried in St Peter's at Doddington, Lincolnshire, instead. The mausoleum stands ruinous in the park now, its lead roof gone, a monument to a young man and to a particular kind of 18th-century hubris that built a whole tomb for a nineteen-year-old's catastrophic seduction attempt.

The 1822 fire left the house a shell. The architect John Dobson partially restored it between 1862 and 1863, re-roofing the central block, though it remained empty inside. The hall opened to the public for the first time in 1950. The house stood unoccupied until the 1980s, when after 160 years a member of the Delaval line moved back in - Edward Delaval Henry Astley, 22nd Baron Hastings, took the west wing. The National Trust took ownership after a successful public appeal in December 2009. A 3.7 million pound refurbishment funded by the Heritage Lottery Fund began in 2018. In 2021 the hall received funding from the Culture Recovery Fund. Legend says a White Lady can sometimes be seen at a first-floor window on the north front - a girl who fell in love with a Delaval heir and died of a broken heart because the marriage was forbidden. Most great houses have such a story. Seaton Delaval, after the fire and the laundry maid and the Admiral's fall, has earned one or two.