Mark Millar MBE (/ˈmɪlər/; born 24 December 1969) is a Scottish comic book writer who first came to prominence with a run on the superhero series The Authority, published by DC Comics' Wildstorm imprint. Millar has written extensively for Marvel Comics, including runs on The Ultimates, which has been called "the comic book of the decade" by Time magazine and described as a major inspiration for the 2012 film The Avengers by its co-story creator Zak Penn, X-Men, Fantastic Four and Avengers for Marvel's Ultimate imprint, as well as Marvel Knights Spider-Man and Wolverine. In 2006, Millar wrote the Civil War mini-series that served as the centrepiece for the eponymous company-wide crossover storyline and later inspired the Marvel Studios film Captain America: Civil War. The "Old Man Logan" storyline, published as part of Millar's run on Wolverine, served as the inspiration for the 2017 film Logan.

Millar has written numerous creator-owned series which have been published under the unified Millarworld label, including Wanted with J. G. Jones, Hit-Girl & Kick-Ass with John Romita Jr., Nemesis with Steve McNiven, Superior and Supercrooks with Leinil Francis Yu, The Secret Service with Dave Gibbons and Jupiter's Legacy with Frank Quitely. Some of these series have been adapted into live-action and animated series and feature films, such as Wanted, Kick-Ass, Kingsman: The Secret Service, Jupiter's Legacy and Super Crooks, as well as videos games like Wanted: Weapons of Fate and Kick-Ass: The Game.

In addition to his work as a writer, Millar serves as an executive producer on all film and television adaptations of his comics. Between 2012 and 2016, he was employed by 20th Century Fox as a creative consultant for adaptations of his Marvel comic series Kick-Ass and The Secret Service (Kingsman).

In 2013, Millar was appointed a Member of the Most Excellent Order of the British Empire (MBE) for services to film and literature.

Millar was born on 24 December 1969 in Coatbridge, Scotland. He spent the first half of his life in the town's Townhead area and attended St. Ambrose High School.

Millar has four older brothers, and one older sister, who are 22, 20, 18, 16 and 14 years older than him, respectively. He was first introduced to comic books at age 4 by his brother Bobby, who at the time was attending university and, as of 2010, worked at a special needs school. The first comics that Millar read were the seminal The Amazing Spider-Man #121, which featured the death of Gwen Stacy, and a Superman book purchased by Bobby that day. Millar's interest in the medium was further cemented with the black-and-white reprints of other comics that his brothers purchased for him, which he enjoyed so much that he drew a spider web across his face with an indelible marker that his parents were unable to scrub off in time for his First Communion photo a week later.

In the mid-late 1970s, Millar frequently appeared as a guest on the long-running Scottish kids TV programme Glen Michael's Cartoon Cavalcade, which he was a regular fan of at the time. On one occasion, he was invited onto the show to talk about the history of comics and, in a 2010 interview with the Scottish newspaper Daily Record, Millar has stated that Glen Michael's TV programme was where he first discovered superheroes.

Millar's mother died of a heart attack at age 64, when Millar was 14, and his father died four years later, aged 65. Although Millar enjoyed drawing comics, he was not permitted to go to art school because his family frowned upon such endeavours as a waste of time for the academic Millar, who studied subjects like chemistry, physics and advanced maths. He initially planned to be a doctor, and subsequently decided that becoming an economist would be a viable alternate plan, but later decided that he "couldn't quite hack it" in that occupation. He attended Glasgow University to study politics and economics, but dropped out after his father's death left him without the money to pay his living expenses.