Johnston was the third of seven children born to an Air Force sergeant and his music-teacher wife. She received a B.A. in theatre arts from Jacksonville University in 1970, then earning an M.A. in theatre from the University of Illinois, Urbana in 1971. She received a law degree (with honors) at the University of Texas at Austin in 1980. For the next five years, Johnston worked as an attorney, serving with the Hunton & Williams firm in Richmond, Virginia, and with Squire, Sanders, & Dempsey in Miami. She has also worked as a newspaper editor and drama critic in San Antonio, Texas, and as a college professor at Southwest Texas Junior College, Barry University, and the University of Miami.
Johnston is a member of the Authors Guild, Novelists, Inc., Romance Writers of America, and Florida Romance Writers. She has two children and one grandchild, and divides her time between two homes, in Colorado and Florida.