The legend of a knight called Edmund coming from Hungary with Queen Margaret, wife of Malcolm III, cannot be substantiated. More factually it seems almost certain that the Edmonstones descend from the Setons, a family of Flemish extraction, who held extensive lands in the Lothians. Most surnames originally stemmed from a designation. The word tun or ton signified a town and therefore it seems probable that an Edmund, of the race of Seton, received the lands of Edmonstone in Midlothian, in the parish of Newton, four miles to the east of Edinburgh, as his appendage. The similarity of the three crescents which appear in the coats of arms in the Edmonstones of Duntreath and of the Setons of Abercorn, near Linlithgow, give added evidence of relationship between the families concerned.

The fact that there is a place called Edmonstone in Lanarkshire is confusing, but no documentation has as yet been found to prove its connection with the senior line of the family.

The name is first recorded in 1248 when Henricus de Edmundiston was witness to a charter. In 1359, in the reign of David II, an inquest before the Baillie of Musselborough, declared that "Henricus de Edmundiston" had died and that "Johannes de Edmundiston" was his legitimate son and heir. It added that he held land of the Abbey of Dumfermline.

John Edmonstone, in 1352, was appointed by charter coroner to the district of Lothian. In 1363 he joined the escort which accompanied David II to England to negotiate a truce. In 1367 and again in 1369 passports were issued to Sir John and other knights, to travel to England on the King's behalf. The truce concluded at Edinburgh Castle, on the 20th July 1369, was signed by John de Edmondiston Miles and others of the chief nobility. Excerpt from the Edmonstone Family website.