Gygax co-authored Chainmail, a set of medieval miniature wargaming rules, with Jeff Perren. Chainmail served as a precursor and a key element in the development of Dungeons & Dragons, especially its combat system.

Gygax encountered Dave Arneson and his game, Blackmoor, a medieval combat game that incorporated elements of fantasy role-playing. This meeting proved pivotal, leading to the collaboration that would ultimately create Dungeons & Dragons.

Gygax and Arneson worked together to refine and expand upon the concepts of Chainmail and Blackmoor, resulting in the creation of Dungeons & Dragons. This groundbreaking game combined wargaming, fantasy storytelling, and player-driven narratives.

The first edition of Dungeons & Dragons was published as a boxed set by Tactical Studies Rules, Inc. (TSR). The initial print run was limited, but the game quickly gained popularity through word-of-mouth and grassroots marketing.