Sundrum Castle is amongst the oldest inhabited castles of Scotland, dating back to the wars for Scottish Independence when it was declared forfeit to the crown.

Sir Robert Wallace, a relative of Sir William Wallace, the Scottish freedom fighter, was appointed Sherriff of Ayr in 1342, succeeded by his son Duncan
in 1359 who commissioned the building of the present castle in the 1360's.

The Tower was originally built with only narrow slits for windows to light the ground floor and the basement only. The entrance to the castle was in the north wall and a corridor within the thickness of the wall led through the guardroom to the Great Hall. The guardroom still exists, below which lies a small pit prison now sealed up. A wooden screen divided the Great Hall, where all the administrative business of the Sheriffdom was undertaken. The floor above was reserved as the Sheriff and his Lady's private quarters.

With the consent of King Robert II, possession of the castle devolved to Sir Alan de Cathcart before 1384, the consequence of Sir Duncan dying without an heir. Sir Alan's father, a supporter of Robert the Bruce, is recorded as having been present at the battle of Loudon Hill. In Barbour's history of the Bruce, he is noted as "amongst a party of 50 knights led by Sir Edward Bruce who surprised and defeated a far superior number of English knights in Galloway ".

The same Sir Alan famously accompanied Sir James Douglas on the ill-fated quest to take the heart of Robert the Bruce on crusade to the Holy Land. Surviving a bloody battle against the Moors in Andalusia, he and Sir William de Keith retrieved the heart in its casket from the battlefield and brought it back with the bones of their dead comrades to Scotland. The heart was buried in Melrose Abbey. The arms of Cathcart have made reference to this event ever since. In 1996 the lead casket was rediscovered at Melrose and re-interred.

The castle remained the Cathcart family seat until 1753, when it was sold by the 9th Lord Cathcart for 18,000 to John Murray of Broughton.

Within a few years it was purchased by the Hamiltons, with whom it remained until 1917.

In the 1790's the Hamiltons carried out major alterations to Sundrum adding the rear west portions and northern extensions, establishing the Mews and its characteristic Clock Tower. John Hamilton of Sundrum was instrumental in reducing the claim for rent against William Burns, the father of Robert Burns helping to alleviate the family's plight. John Hamilton married Lillias Montgomery, sister to the 12th Earl of Eglinton and their entwined crests are carved on the Great Hall fireplace.

A further four generations of Hamiltons continued to reside and improve the castle until 1917 when it was sold to Mr. Earnest Coats, a director of the Paisley firm of thread manufacturers. The area now known as the "Coats House" was added between the mews and the original castle.

In 1936, Sundrum was sold to an enterprising hotel syndicate. In subsequent years the castle fell into disrepair, before award-winning restoration and development of the estate was undertaken by Salopian Estates assisted by Historic Scotland.

Patricia and Graham Cathcart Waddington have undertaken the interior refurbishment of the tower itself.