Mark Gruenwald explores the career and lasting influence of Mark Gruenwald (1953-1996), a pivotal figure in the history of Marvel Comics. Known for his humor, encyclopedic knowledge of superhero lore, and dedication to continuity, Gruenwald played a critical role in shaping Marvel's universe and storytelling during his nearly two decades at the company. This biography traces Gruenwald's journey from a comic-obsessed child and fanzine creator to a prolific writer, editor, and eventually Marvel's executive editor. His creative achievements include groundbreaking works like Squadron Supreme, editing popular comics such as the Avengers, Iron Man, and Thor during creative high points, innovative contributions to Quasar and Captain America, and leading the production of The Official Handbook of the Marvel Universe. Gruenwald's legacy also extends to the rules for time travel and alternate realities he established, which continue to influence the Marvel Cinematic Universe today. Gruenwald's influence resonates beyond comics, inspiring elements of major Marvel films and TV series such as Avengers: Endgame, Loki, and The Falcon and the Winter Soldier. This first-ever full account of his life and work underscores his vital contributions to superhero storytelling and affirms his lasting relevance to fans and creators alike.