Clan Johnstone consists of at least two distinct groups, those of Annandale in Dumfries and Galloway, and those of Caskieben in Aberdeenshire, as well as Johnston(e)s who cannot easily be classified as either, such as those who took their name from the former name of Perth (St. Johnstoun) or other places called Johnston or Johnstone.

A word about pronunciation and spelling: Americans and Canadians tend to pronounce Johnstone as "Johnstoan" and Johnston as "Johnston", suggesting two different names. In the U.K. generally, the name is pronounced "Johnston", no matter how it is spelled; native border Scots often pronounce it "Jawnson." Presently, the Chief of the entire Name spells his name "Johnstone," and the Head of the House of Johnston of Caskieben spells his name "Johnston." Actually, either spelling may be used by Annandale or Caskieben clansmen, although the "e" is more common among those of Annandale. The writer has tended to use the "e" in this short history because it is the way the Chief of the entire Clan spells his name, and also because that is the way the writer spells his name.