Mark Eugene Gruenwald (June 18, 1953 – August 12, 1996) was an American comic writer, editor, and occasional penciler known for his long association with Marvel Comics. Gruenwald got his start in comics fandom, publishing his own fanzine Omniverse, which explored the concept of continuity. His first professional gig was writing text articles for DC Comics official fanzine, The Amazing World of DC Comics and in 1978 he was hired by Marvel Comics, where he remained for the rest of his career. Hired initially as an assistant editor in January 1978, Gruenwald was promoted to full editorship by Marvel editor-in-chief Jim Shooter in 1982, putting Gruenwald in charge of The Avengers, Captain America, Iron Man, Thor, Spider Woman, and What If?. Gruenwald also did fill-in pencil work for a handful of Marvel comics, most notably the Hawkeye limited series, but also issues of What If? Marvel Team-Up Annual, The Incredible Hulk, and Questprobe. In 1982 Gruenwald famously co-wrote Marvel Superhero Contest of the Champions, the first limited series published by Marvel Comics. 

As a writer, Gruenwald is best known for creating the Official Handbook of the Marvel Universe and his ten-year stint as the writer of Captain America (from 1985 to 1995) – during which he contributed several notable characters such as Crossbones, Diamondback and John Walker- U.S. Agent. He made a deliberate effort to create villains who would be specific to Captain America, as opposed to generic foes who could as easily have been introduced in another comic. At one point Gruenwald owned a replica of Captain America's shield – the same shield now owned and displayed on the set of Stephan Colbert. His 60-issue run on Quasar (1989–1994) realized Gruenwald's ambition to write his own kind of superhero, however he considered his greatest achievement to be the mid-1980s 12-issue miniseries Squadron Supreme which tells the story of a well-intended group of superheroes in an alternate universe who decide that they would be best suited to run the planet. Gruenwald was highly loyal to each series he wrote and in addition to his lengthy stint on Captain America, he wrote the entire runs of both Quasar (save for one issue) and D.P.7.

On September 1, 1987, he became Marvel's executive editor, with a particular remit as the keeper of continuity. Gruenwald was famous for a perfect recollection of even the most trivial details. In creating the Time Variance Authority, a cosmic bureaucracy that regulates the Marvel Multiverse writer Walt Simonson paid homage to Gruenwald by having the TVA staffed by clones of Gruenwald and his likeness is featured in the Disney + Series Loki. No one could keep track of everything but Mark Gruenwald ("Gru" or "Grueny" as he was often referred to). 

In 1996, Gruenwald succumbed to a heart attack, the result of an unsuspected congenital heart defect. A longtime lover of comics, Gruenwald made it known amongst his friends and families that his one desire was to have his ashes used in part of a comic. In accordance with his request, he was cremated, and his ashes were mixed with the ink used to print the first printing of the trade paperback compilation of Squadron Supreme. He is survived by his Wife Catherine Schuller Gruenwald and his Daughter Sara.