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's Lucasfilm Ltd. company. Johnston began working at the company, eventually promoted to a head storyboard position that 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 original 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.
During pre-production for Star Wars: Episode V The Empire Strikes Back, Johnston was responsible for the design of Boba Fett's distinctive armor and equipment. He worked alongside artist Ralph McQuarrie to develop the unique look for the bounty hunter, which was in fact derived from the early concepts of Darth Vader.
Johnston on the set of Star Wars: A New Hope being directed by George Lucas.
Johnston played an Imperial gunner and a backpacked spacetrooper in Star Wars: Episode IV A New Hope, and in that costume he was made into a Hasbro action figure in 2009.
In July 2011, Johnston stated that he was "trying to get George [Lucas] to make a feature based on Boba Fett.
- (1977) (uncredited role as an Imperial gunner, effects illustrator and designer: miniature and optical effects unit)
- (1980) (uncredited role as Shawn Valdez, art director visual effects)
- (1983) (art director visual effects)
- Caravan of Courage: An Ewok Adventure (1984) (production designer)
- Ewoks: The Battle for Endor (1985) (production designer)
- Star Wars: Droids: The Adventures of R2-D2 and C-3PO — "Coby and the Starhunters" (1985) (co-writer)
- From Star Wars to Jedi: The Making of a Saga (1985) (interviewed)
- Star Wars: Droids: The Adventures of R2-D2 and C-3PO — "The Great Heep" (1986) (design consultant)
- Empire of Dreams: The Story of the Star Wars Trilogy (2004) (interviewed)
- The 100 Greatest Family Films (2005) (interviewed)
- The Adventures of Teebo: A Tale of Magic and Suspense (1984) (writer)
- The Star Wars Sketchbook (writer & illustrator)
- The Empire Strikes Back Sketchbook (writer & illustrator)
- Return of the Jedi Sketchbook (writer & illustrator)
- Raiders of the Lost Ark (1981) (art director: visual effects, ILM)
- Indiana Jones and the Temple of Doom (1984) (art director: California)
- Howard the Duck (1986) (ultralight sequence design)
- Willow (1988) (associate producer)
- The Young Indiana Jones Chronicles (1993) (director of "Princeton, February 1916" episode)