Yves de Vescy, Baron of Alnwick, erected the first parts of the castle in 1096. It was built to defend England's northern border against the Scottish invasions and border reivers.
It was besieged in 1172 and again in 1174 by William the Lion, King of Scotland and William was captured outside the walls during the Battle of Alnwick.
In 1309 it was bought from Antony Bek the Bishop of Durham by Henry de Percy, 1st Baron Percy and it hassince then been owned by the Percy family, the Earls and later Dukes of Northumberland . The first Percy lord of Alnwick restored the castle and the Abbot's Tower, the Middle Gateway and the Constable's Tower survive from this period. In 1404–5 the Percys rebelled against Henry IV, who besieged and then took the castle.
During the Wars of the Roses it was held against King Edward until its surrender in mid-September 1461 after the Battle of Towton. Re-captured by Sir William Tailboys during the winter he surrendered to Hastings, Sir John Howard and Sir Ralph Grey of Heton in late July 1462. Grey was appointed captain but surrendered after a sharp siege in the early autumn. King Edward responded with vigour and when the Earl of Warwick arrived in November Queen Margaret and her French advisor, Pierre de Brézé were forced to sail to Scotland for help. They organised a mainly Scots relief force which, under George Douglas, 4th Earl of Angus and de Brézé, set out on 22 November. Warwick's army, commanded by the experienced Earl of Kent and the recently pardoned Lord Scales, prevented news getting through to the starving garrisons. As a result the nearby Bamburgh and Dunstanburgh castles soon agreed terms and surrendered. But Hungerford and Whittingham held Alnwick until Warwick was forced to withdraw when de Breze and Angus arrived on 5 January 1463.
The Lancastrians missed a chance to bring Warwick to battle instead being content to retire, leaving behind only a token force which surrendered next day.
By May 1463 Alnwick was in Lancastrian hands for the third time since Towton, betrayed by Grey of Heton who tricked the commander, Sir John Astley. Astley was imprisoned and Hungerford resumed command.
After Montagu's triumphs at Hedgeley Moor and Hexham in 1464 Warwick arrived before Alnwick on 23 June and received its surrender next day.
The 6th Earl of Northumberland carried out renovations in the 16th century. In the second half of the 18th century Robert Adam carried out many alterations. The interiors were largely in a Strawberry Hill gothic style not at all typical of his work, which was usually neoclassical.
In the 19th century Algernon, 4th Duke of Northumberland replaced much of this with less ostentatious architecture designed by Anthony Salvin. According to the official website a large amount of Adam's work survives, but little or none of it remains in the principal rooms shown to the public, which were redecorated in an opulent Italianate style in the Victorian era by Luigi Canina.
Since the Second World War, parts of the castle have been used by various educational establishments: Firstly, by the Newcastle Church High School for Girls then, from 1945 to 1975, as a teacher training college and, since 1981, by St. Cloud State University as a branch campus forming part of their International Study Programme.