From the 12th century the manor of Muchland — later known as Aldingham — was administered from Aldingham Castle on the coast. In 1291 the manor passed to the Harington family. Then two things went wrong. The Scots came down through the Furness peninsula during the Wars of Scottish Independence, raiding into the early 14th century. And the sea, which had always sat too close to Aldingham Motte, began to take it. The two pressures together pushed the Haringtons inland. They picked a spot about a kilometre northwest of Gleaston village, where a small valley offered shelter and a stream, and they built a new administrative seat. The castle was probably built for John Harington, 1st Baron Harington, who lived from 1281 to 1347 and would have wanted a residence equal to his rising status. He had been licensed to enclose a 600-acre park nearby, the kind of thing a lord did when his servants and ambitions both grew.

Gleaston Castle is first mentioned in a document of 1389, though John Harington's son, the 2nd Baron, is said to have died here as early as 1363. In 1415 another John Harington was granted a papal indult — a special permission — for a private chapel and a portable altar to celebrate mass. It is likely the castle had always had a chapel of its own. In the 19th century, when farm buildings were being put up against the castle's eastern wall, builders dug into the ground and found up to four human skeletons. A 1905 document at the Cumbria Archive Centre records the discovery without explaining who they were. They could have been soldiers from one of the unrecorded skirmishes the Scots brought south. They could have been a forgotten chapel burial. The names are gone, but the bones were once people who lived here, served here, perhaps died here.

In 1458 the castle passed by marriage from the Haringtons to William Bonville. The Bonvilles seem not to have cared for it, and the castle was effectively abandoned. It passed eventually to the Grey family, until Henry Grey, 1st Duke of Suffolk — father of the tragic nine-day queen Lady Jane Grey — was executed for treason in 1554. Gleaston became royal property, was sold to the Prestons in the 17th century, and eventually came into the hands of the Cavendish family. Throughout, it sat empty. By the time Samuel and Nathaniel Buck made one of the earliest engravings of the castle in 1727, Gleaston was already a ruin. William Close drew it again in 1805. William Green in 1809. Edwin Waugh in 1860. Each image shows roughly the same skeleton — and that skeleton is essentially the silhouette walkers see today from the roadside.

Gleaston Castle is now a Grade I listed ruin and a scheduled monument, but it is on Historic England's Heritage At Risk register, where it has sat since at least 2016. The condition is described as "very bad" with "immediate risk of further rapid deterioration or loss of fabric." It is unsafe to enter; visitors view it from the road. In 1998 the Lancaster University Archaeological Unit assessed whether the building could be opened to the public, but no consolidation scheme was agreed. Then in 2015 the Morecambe Bay Partnership, with funding from the Castle Studies Trust, commissioned Greenlane Archaeology to perform an aerial survey — the first time the castle had ever been fully recorded. A 2016 geophysical survey by the University of Central Lancashire, run partly as training for archaeology students, revealed traces of a vanished garden to the north and timber structures within the walls. The red sandstone for the dressings was probably hauled from a beach 3 km away, the same building trick used at Piel Castle near Barrow. After six centuries of weather, the western curtain wall still stands 9 metres tall in places.