The lands of Sorn belonged originally to the Keiths of Galston befpre passing to the Hamilton family, confirmed by a Charter under the Great Seal on the 11th December, 1406.  Sir William Hamilton of Sorn and Sanquhar, a son of this marriage, was one of the Senators of the College of Justice and Lord Treasurer to King James V, as well as Captain of Edinburgh Castle. His daughter Isobel Hamilton, was his heiress who married George, 7th Lord Seton, and became mother to Robert, 1st Earl of Winton; to Alexander, 1st Earl of Dunfermline; to Sir John of Barnes; to Sir William of Kyllismuir; and to Margaret, the wife of Claud Hamilton, Lord Paisley, ancestor to the Earl of Abercorn.

James V visited Sorn Castle on the occasion of the marriage of Isobel Hamilton, the daughter of his Lord Treasurer with George Seton, son of the 6th Lord Seton one of his Lords of Justice.  The chair which His Majesty is said to have used on the occasion was kept in Sorn Castle till the sale of the estate in 1782, when it was transferred to Loudoun Castle, where it still remains.

The marriage of the first Isobel was a very important one which, "the union was devised to bring about an alliance betwixt the Setounes and the Governor Arran," to whose house Sir William belonged, and was of such political importance that a medal commemorating it was struck, bearing the initials of the bride and bridegroom, I. H. and G. S., and the motto, "ung loy, ung foy, ung roy," is of great historical interest, as an example of ancient phonetic spelling of French!