As the Domesday Book did not extend to Northumberland, and the earlier records do not recount the castles creation, the history of the castle can only be traced from the time of the Norman Conquest. Gilbert Tyson or De Tesson, standard bearer to William at the Battle of Hastings was the first known owner of Alnwick although there is no record of whether the castle had already been in existence.

Tyson was eventually deprived of the lands owing to his involvement in the death of Malcolm Canmore, King of Scotland, and in 1096 the castle was owned by Yvo de Vescy. It passed through several generations of this family until the death of William de Vescy who had placed the castle in trust until his illegitimate son William came of age. The Bishop of Durham, unscrupulous as he was, sold the entire estate to the de Percy family in 1309. William de Vescy was eventually killed at the Battle of Bannockburn in 1314.

The castle became the family seat of the Percy's, and when Henry Percy was made Earl of Northumberland in 1368, it became the seat of the Earldom, and later the Dukedom. In 1585 Henry Percy became the 9th Earl of Northumberland and succeeded in gaining great influence both in the court of Elizabeth, and in her successor, James VI of Scotland, however in 1594, he appointed a relative Thomas Percy, Constable of Alnwick and his Commissioner and Auditor. Through his connections, Thomas, known as an 'outspoken and bigoted Catholic', and accused of harsh and unjust conduct, became a spokesperson to the Scottish King on behalf of the English Catholics. Thomas and his family lived for many years at Alnwick, but the succession of James brought new hope.

Alas, it was not to be, and James soon reneged on the promises he had made to Thomas Percy regarding toleration for the English Catholics. Percy, a close friend of Robert Catesby, was soon drawn into the folds of the gunpowder treason. After his death, the Earl was tainted with the stain of supposed confederacy with Thomas, and although nothing was ever proven, the Earl spent many years incarcerated in the Tower.