I was not able to find a site to give me a lot of historical information on the Johnstone surname origin. I will post what I have found and suggest that anyone with more information forward it to me.

Spelling variations include: Johnston, Johnson
Rob Wermuth was conducting research into the Johnson part of the family tree and came across an entire web-site with Johnson/Johnstone information. Johnston Clan This may be a good resource for additional Johnson family information.