Known for designing many state prisons and insane asylums in Minnesota, such as the Minnesota Security Hospital, several wings at the Minnesota Correctional Facility, and the Minnesota State Academies for the Blind and the Deaf.
He first worked for Abraham M. Radcliffe's firm as a draughtsman. He had to drop out of MIT because of financial restraints.
He was the state architect of Minnesota until 1931.
His parents, Alexander and Louise Johnston, helped build a settlement in Waseca County, Minnesota, in 1856 called Okaman, which was next to Lake Elysian. His son, Clarence Johnston Jr., also became an architect.
He and Frank Gehry both became famous architects, albeit across different generations.