Clan Boyd came into possession of the lands of Dean in 1316, when Sir Robert Boyd was rewarded by King Robert I for his service at Bannockburn. The lands of Kilmarnock and West Kilbride were the prize for backing the Bruce in 1314. Around 1350 Sir Robert's son, Sir Thomas Boyd, built the Keep that still stands at the heart of the complex. The walls are two to three metres thick. The windows are few. The original entrance sits high above ground level, reached by a removable wooden stair. Most striking of all, there are no arrowslits in the walls, which is rare for a Scottish castle of that period. All defensive shooting was done from the battlements at the top. The Boyds clearly believed in defence by elevation and mass, not by archery loops cut into walls. The Palace was built alongside the Keep in the mid-1300s, the joint creation of Sir Robert and Sir Thomas.

In 1735 an accidental kitchen fire spread to the thatched roof of the Palace and from there to the Keep. The castle was reduced to a complete ruin and entered nearly two centuries of neglect. William Boyd, the 4th Earl of Kilmarnock and the unfortunate resident at the time, was in financial trouble and could not afford to repair it. Eleven years later he would be beheaded in London. Some buildings on the estate remained in use, but the castle itself stood as a roofless shell into the 20th century. In 1975 the 9th Lord Howard de Walden gave the castle, the surrounding 200-acre estate, his father's collection of European arms and armour, and his grandfather's collection of early musical instruments to the people of Kilmarnock. The collections are now displayed in the Great Hall (arms) and the solar (instruments). Between 2020 and early 2023, Dean Castle underwent a £5.2 million restoration, funded partly by the National Lottery Heritage Fund and Historic Environment Scotland and partly by East Ayrshire Council, with extensive pointing work and structural repairs.

The Keep has four floors. The ground floor held a cellar and the original kitchen for the Great Hall above; it also held the bottleneck dungeon, a small chamber accessed only via a hole in the guard-room floor on the floor above. Prisoners were dropped in. There were no other doors. On the first floor sits the Great Hall, vaulted ceiling stretching high overhead. This was the room of grand banquets, of guest sleeping arrangements, of the lord acting as judge over crimes committed in the surrounding lands. The first floor also held the minstrel's gallery, where travelling musicians played for the lord and lady below. Off the gallery is the minstrels' changing room, where the travelling players also slept; the practical reason being a fear that they might carry disease and were better kept apart from other guests. The second floor held the solar, the private chambers, divisible into separate halves for ladies and gentlemen by a large curtain. A small private chapel adjoined the solar. The third floor, on top of the keep, held archers and a small set of apartments for soldiers.

Dean Castle Country Park covers 200 acres of grounds open to the public. There are cafes, play parks, a Rural Life Centre, and a Visitor Centre. Illuminight, a light show created in 2017, drew more than 80,000 visitors in its second year, and Spirit of Christmas, launched in 2022, projects festive light onto the castle facade through the winter season. The Robert Burns World Federation unveiled a plaque to the victims of the September 11, 2001 attacks, carrying a line of Burns: "Man's inhumanity to man makes countless thousands mourn!" In Outlander season two, Dean Castle stood in for Beaufort Castle, seat of Clan Fraser of Lovat. A large bust of William Wallace sits on the ground floor of the Palace. The Kilmarnock Edition of Burns's poetry is displayed in the banqueting hall. Seven centuries after Bruce gave the Boyds the land, Dean Castle has settled into the unusual role of a working medieval museum, a free public park, and a place where tourists come to look for ghosts.