Johnson was born in Aurora, Illinois, in 1940. He received a BA degree in English literature, from Harvard University in 1961. He studied law at the University of Chicago, graduating top of his class, and received a Juris Doctor in 1965. He served as a law clerk for the Chief Justice of the U.S. Supreme Court Earl Warren and Chief Justice of the California Supreme Court Roger J. Traynor. Johnson became a member of the California Bar in January 1966. From 1967 to 2000, Johnson was an active professor of law at the U.C. Berkeley School of Law and retained the title professor emeritus at the time of his death. Johnson served as deputy district attorney and held visiting professorships at Emory University and at University College London.