The Estate of Thornton lies at the northern end of the Vale of Strathmore in the fertile Howe o' the Mearns, an area of one time strategic importance since it commanded the approach to the Cairn-a-Mounth pass over the Grampians to Deeside and the North. The castle, which lies midway between the market town of Laurencekirk and the village of Fettercairn, is best approached by the North Lodge entrance at Thornton cross roads on B9120 some 2 1/2 miles from each of these places. A visitor approaching the north-west front of the castle by this avenue will see on the left a squat round tower carrying the arms of the Strachans of Thornton and shot holes commanding the front elevation; the base of this tower, constructed of great whin-stone boulders, is a survival of an earlier building which stood on the site and extended over part of the gravel forecourt in the 14th century. On the 1st floor of this keep the stone walls of the former great hall, now oak panelled, carried some ancient mural paintings and in one of the deep window recesses a small hinged panel can be opened to disclose a tempera portrait with the inscription "Dame Elizabeth, Lady of Thornton." This was Elizabeth Forbes, wife of Sir James Strachan of Thornton, to whose memory there was an elaborate memorial in the Thornton aisle in the Marykirk parish church and who died in childbirth in her 25th year in 1661. A window lintel above the present entrance bears the date 1662 but a stone achievement below this was added by Sir Alexander Strachan soon after his appointment as one of the 3 original baronets of Nova Scotia in 1625. Early photographs show that the exterior of the castle, with the exception of the battlements and corbelling and the massive buttresses, was at one time faced in white. Today the stonework, with its blue and grey mountain boulders and the contasting pink of the sandstone, can be seen in the warmth of its original texture. The parks in front of the castle and the gardens, now much reduced in size, are studded with fine old trees notable among which are two ancient yews reputed to be over 600 years old.
There are records of Thorntons of that Ilk in the Mearns from the 13th century but the male line failed in the reign of David II and the lands and barony of Thornton passed to the Strachan family with the marriage of Sir James Strachan to Agneta, heiress of Valentine de Thornton who had received a charter "de terris de Thornetoun in lie Kincardin-Mernis" from Robert the Bruce in 1309. Thereafter for over 300 years Thornton was in the possession of the "ancient family of Strachan" - a family which was ardently devoted to the service of the Crown. David Strachan, 9th of Thornton, was Justice Clerk from 1492-1497. The 11th laird, John Strachan - whose daughter Elizabeth married William Forbes of Corse and bore him six distinguished sons including John Forbes. A granddaughter of this laird, Catherine Strachan of Thornton, was mother of the Covenanting General John, Earl of Middleton.
But not all Strachan lairds were models of propriety. Sir Alexander Strachan, a Commissioner of the Exchequer and 1st baronet of Thornton, scandalised the whole country when he conveyed the gladly consenting Margaret Ogilvy, Countess Marischal, and with her a priceless dowry of plundered heirlooms, jewelery and furnishings, to live with him at Thornton while her famous husband, George Keith, 5th Earl of Marischal, lay dying in his sea-girt stronghold of Dunnottar.
The only surviving son of this infamous laird, Colonel Alexander Strachan, was a distinguished cavalry officer who held Thornton for the Covenant against the forces of the Marquis of Montrose in 1646, for which service "and for the benefit of the country round about" he was granted a provision of arms, ammunition and victuals for a garrison of thirsty men. After the regicide, like many another disillusioned Parliamentarian, he transferred his allegiance to Charles II, by whom he was appointed in 1650 a Commissioner to advise the King upon the government of Scotland and the administration of the Army.
With the marriage in 1654 of Sir James Strachan, 3rd baronet and 17th laird, to his 16 year old cousin Elizabeth Forbes of Waterton and of their son James (who fell by the side of his leader "Bonnie Dundee" at the battle of Killiecrankie) to Barbara Forbes of Newton the estate passed into the successive ownership of 3 Forbes lairds and, in 1720, to Thomas Fullerton of Gallery, the initials of whose son, Phillip (Beau) Fullerton of the Dragoon Guards and of his wife Katharine Scott of Benholm embellish the gilded weather-vane on the keep.