Johnson played his first game of Dungeons & Dragons in 1976; he soon became an avid player, and attended his first Gen Con convention in Lake Geneva in 1977.

After graduating from college, Johnson continued to work odd jobs, and play games.

[1] Johnson supervised all of the staff game designers for TSR, and also managed the company's Acquisition Department.

Johnson was responsible for hiring many new game designers during his tenure with TSR, including Frank Mentzer, Jon Pickens, and Tracy Hickman.

[2] Harold Johnson designed the modules The Hidden Shrine of Tamoachan (1980), and Secret of the Slavers Stockade (1981), and also wrote the IJAC1 Judge's Survival Pack, and IJ4 The Golden Goddess for the Indiana Jones role-playing game.