In the 1970s, Mitchell wrote a seminal book on the use of computers in architecture titled Computer-Aided Architectural Design.
M.I.T. professor George Stiny is quoted in the NY Times: "If he hadn't been there to inaugurate computer-aided architectural design, architects would probably still not be doing it. Remember, in 1977 it was hard to draw a line on a computer. Bill really had a sense of how much architects could take, gave them a little more, and made it possible for them to take the next step."
More recently, Mitchell was involved in the development of designs for stackable cars and scooters that could change the way people think of personal transportation.
A more personal recollection of Mitchell can be read here, from Planetizen contributor Anthony Townsend, a former student of Mitchell's.