He attended and received degrees from the University of Virginia and the University of Alabama. In 1905 he married Clara Vernon Cocke of Virginia and they had three children: Joseph Forney, Virginia, and Paul.
Forney was a senior member of the law firm Cabaniss and Johnston, and also served as general counsel for the Seaboard Airline Ry. and the National Association Owners of R.R. Securities; and on the Board of Trustees for the First National Bank of Birmingham and the YMCA of Birmingham.
Johnston was also influential in a wide variety of political and legal matters. He drafted the basic rate-making principals in the Transportation Act of 1920, represented the plaintiffs in the Ashwander case that attacked the constitutionality of the act creating the Tennessee Valley Authority, denounced the Ku Klux Klan at the 1924 Democratic Convention when nominating U.S. Senator Oscar Underwood for the presidential nomination, and published several legal papers on the constitutionality of government policies during the Depression and afterwards.