The original Norman castle, was built in about 1122 by Geoffrey de Clinton, Chamberlain to King Henry I. The castle consisted of a simple round enclosure defended by an earth bank and a tall mound topped by a wooden watchtower. By the end of the century most of the castle had been rebuilt in stone, with a large keep encasing the earth mound and replacing the watchtower, with a hall and chapel all inside a protective curtain wall.

Some fifty years after the castle was built it was taken by King Henry II to counter an attack by his son's rebel army. After the rebellion had ended, King Henry II did not return Kenilworth Castle to the de Clinton family, but gave them other lands instead, with a very small castle, in Buckinghamshire.

Between 1210 and 1215 King John paid for extensive improvements to Kenilworth Castle. An outer perimeter wall was built with towers at intervals, and a fortified dam was thrown up to create an enormous shallow lake around the castle.

Kenilworth remained in Royal hands until 1244 when King Henry III gave the castle to his son-in-law Simon de Montfort. In 1266 the de Montfort's, opposed to the absolute power of the monarchy, declared war on the king, making Kenilworth their rebel headquarters. The castle was besieged, but managed to hold out for nearly nine months. Because the castle was protected on three sides by water, the attackers could not undermine the walls and had to concentrate instead on trying to breach the defences using great war machines and catapults. Surrender finally came after disease and hunger had taken their toll.

John of Gaunt rebuilt the Inner Court into a magnificent palace between 1389 and 1394. King Henry V had a pavilion (banqueting or pleasure house) built at the far end of the Great Mere which he called Le Plesaunz en Marys (the pleasure house in the marsh). During his reign, King Henry VIII moved the Pleasuanz buildings back to the castle site, adding an extra range of lodgings in the Inner Court.

From 1563 Kenilworth was owned by Robert Dudley, Earl of Leicester, who was a great favourite of Queen Elizabeth. He built a fine gatehouse and a graceful residential suite, intended specifically for the Queen. She often visited the castle where she would be lavishly entertained, at great expense.

After Robert Dudley's death in 1588, Kenilworth Castle was claimed by the Crown and eventually sold for a very low price, becoming part of Queen Henrietta Maria's marriage-portion in 1626. After the Civil War, in which it played only a minor part, it was partially demolished by Parliamentary troops. Over the years it was allowed to fall further into ruins and the lake drained away. The castle was saved for the nation in 1938.