With Firaxis, Johnson helped in programming the popular video gameCivilization III, also writing the entire artificial intelligence (AI). He was also the lead designer of Civilization IV, once again writing all of the AI. On April 17, 2007, it was announced that Johnson had left Firaxis to rejoin Electronic Arts to work on Spore.
He is a bi-monthly design columnist for Game Developer Magazine and also on the Advisory Board of the Game Developer's Conference. His current project is Strategy Station.