Johnston was educated at California State University, Long Beach, as well as the Art Center College of Design. Johnston responded to an ad looking for a storyboard artist at college, which turned out to be for George Lucas' Lucasfilm company. Johnston began working at the company, eventually promoted to a head storyboard position which frequently teamed him with Lucas in the editing room. Johnston later referred to working for Lucas as the best film school he could ever have when he watched Lucas do the editing for the Star Wars trilogy. Eventually, Johnston became disillusioned with his job and decided to leave Lucasfilm with his earnings to travel the world. But Lucas had other ideas when Johnston came to leave his notice. Lucas advised Johnston that he move him to the front line of applicants of his alma mater University of Southern California's Film School with Lucas paying for his tuition and keeping him on at half salary. Johnston said he didn't think twice when accepting Lucas' proposal, which he did in 1984.