Pekar's friendship with Robert Crumb led to the creation of the autobiographical comic book series American Splendor, later adapted as a movie. Crumb and Pekar became friends through their mutual love of jazz records; as a result of his friend's work in underground comics, Pekar says, "he started to see the form's possibilities. "Comics could do anything that film could do," he offers. "And I wanted in on it." But it took Pekar some time to decide just what he wanted to do. "I theorized for maybe ten years about doing comics," he says " Pekar laid out some stories with crude stick figures and showed them to Crumb and another artist, Robert Armstrong. Impressed, they both offered to illustrate, and soon Pekar's story "Crazy Ed" appeared in Crumb's The People's Comics, and Crumb became the first artist to illustrate American Splendor. The comic documents daily life in the aging neighborhoods of Pekar's native Cleveland, where Pekar worked throughout his life (even after gaining fame) as a file clerk in a large Veterans Administration hospital.
American Splendor has been illustrated over the years by some of comics' best talents. Pekar's most well-known and longest-running collaborators include Crumb, Gary Dumm, Greg Budgett, Spain Rodriguez, Joe Zabel, Gerry Shamray, Frank Stack, Mark Zingarelli, and Joe Sacco; while recent years have seen him repeatedly team up with artists like Dean Haspiel and Josh Neufeld. Other notable cartoonists who have worked with Pekar include Jim Woodring, Chester Brown, Alison Bechdel, Gilbert Hernandez, Eddie Campbell, David Collier, Drew Friedman, Ho Che Anderson, Rick Geary, Ed Piskor, Hunt Emerson, Bob Fingerman, and Alex Wald; as well as such unexpected illustrators as Pekar's wife Joyce Brabner and legendary comics writer Alan Moore. Still, Harvey loved collecting, and his interests turned to jazz. Through that obsession, he began writing as a critic in 1959, and among his fellow fans was a young Robert Crumb.
The critically acclaimed film adaptation of American Splendor was released in 2003, directed by Robert Pulcini and Shari Springer Berman. It featured Paul Giamatti as Pekar, as well as appearances by Pekar himself. Columnist Jaime Wolf wrote a laudatory review of the film in Slate, also drawing attention to formal parallels with Woody Allen's Annie Hall and other Allen film's. Pekar wrote about the effects of the film in American Splendor: Our Movie Year.
On October 5, 2005, the DC Comics imprint Vertigo released Pekar's autobiographical hardcover The Quitter, with artwork by Dean Haspiel. The book detailed Pekar's early years, and was created in part to reward Haspiel for his role in introducing Pekar to the producers who made the American Splendor movie.
In 2006, Pekar released a four-issue American Splendor miniseries through Vertigo. This was collected in the American Splendor: Another Day paperback. In 2008 Vertigo released a second "season" of American Splendor that was collected in the American Splendor: Another Dollar paperback.