He teaches and writes on Constitutional Law and related subjects. Before joining the School of Law, Johnstone served as the Solicitor for the State of Montana. He represented the State in more than two-dozen cases in trial and appellate courts, including the Montana Supreme Court and the Supreme Court of the United States. Johnstone also clerked for the Honorable Sidney R. Thomas, United States Court of Appeals for the Ninth Circuit, and practiced litigation as an associate at Cravath, Swaine & Moore in New York, New York. He is an elected member of the American Law Institute. Johnstone holds a B.A. from Yale University and a J.D. with honors from the University of Chicago Law School.