The MacMillans are believed to be of Celtic origin. In the thirteenth century they held lands on Tayside. During the Middle Ages they established themselves in Knapdale by marriage to a McNeill heiress. MacMillan of Knap was Chief of the Clan. A rock at Knap Point in Loch Suibhne bore the inscription "Macmillans right holds good to Knap, so long as wave beats on the rock."
But in 1615 Campbell of Cawdor, on orders from his Chief, the Earl of Argyll, pushed the rock into the sea and the old Macmillan lands have long since been lost to the Clan.
Lieutenant General Sir Gordon Macmillan of Macmillan and Knap, the father of the present Chiet was recognised as Chief in 1951.

Information taken from the MacMillan Clan Heritage Book, available for sale through the site.