Levitz was raised in Brooklyn, New York. during which time he revived the defunct comic news fanzine, The Comic Reader, which according to Levitz, was the first regularly published comics industry news fanzine. Under Levitz's editorship The Comic Reader won two Best Fanzine Comic Art Fan Awards. One of Levitz's teachers, Frank McCourt, was impressed enough with Levitz's work that he arranged for Levitz to appear on McCourt's brother's radio show.
During the course of his research for The Comic Reader, Levitz became well known at the offices of DC Comics, where in December 1972, editor Joe Orlando gave him his first freelance work, initially writing text pages and letter pages, and later working as a per diem assistant editor before writing stories. Levitz later studied business at New York University but had taken no formal education in writing, other than a journalism course. He dropped out after three years in order to concentrate on his writing career.
After serving as Joe Orlando's assistant editor, in 1976 Levitz "fulfilled a lifelong dream" by becoming the editor of Adventure Comics on the eve of his 20th birthday. In 1978, he succeeded Julius Schwartz as the editor of the Batman line of comics.
As a writer, Levitz is best known for his work on the title The Legion of Super-Heroes, which he wrote from 1977–1979 and 1981–1989. Levitz wrote All-New Collectors' Edition #C-55 (1978), a treasury-sized special drawn by Mike Grell, in which longtime Legion members Saturn Girl and Lightning Lad were married. Levitz and artists James Sherman and Joe Staton crafted "Earthwar" a five-issue storyline in Superboy and the Legion of Super-Heroes #241–245 (July–Nov. 1978). He and Keith Giffen produced "The Great Darkness Saga", one of the best known Legion stories, in Legion of Super-Heroes vol. 2, #290–294. Comics historian Les Daniels observed that "Working with artist Keith Giffen, Levitz completed the transformation of Legion into a science-fiction saga of considerable scope and depth." In August 1984, a new Legion of Super-Heroes series was launched by Levitz and Giffen.
With artist Steve Ditko, Levitz co-created the characters Stalker and the Prince Gavyn version of Starman. He wrote the Justice Society series in All Star Comics during the late 1970s and co-created the Earth-2 Huntress with artist Joe Staton. He and Staton provided the JSA with an origin story in DC Special #29. Lucien the Librarian, a character later used in Neil Gaiman's The Sandman series, was created by Levitz and artist Nestor Redondo. Levitz was one of the contributors to the DC Challenge limited series in 1986.
Levitz eventually became an editor, and served as vice president and executive vice president, before assuming the role of president in 2002. In 2006, Levitz returned to writing the Justice Society with issue #82 of JSA, completing that volume before writer Geoff Johns' relaunch.
On September 9, 2009, it was announced that Levitz would step down as president and publisher of DC Comics to serve as the Contributing Editor and Overall Consultant for the newly formed DC Entertainment, and become the writer of both Adventure Comics vol. 2 and Legion of Super-Heroes vol. 6.
Levitz mentioned in an August 2010 interview that he was working on "my first genuine book." His 75 Years of DC Comics: The Art of Modern Mythmaking (ISBN 9783836519816) was published by Taschen America, LLC in November 2010.
In addition to Legion of Super-Heroes, Levitz wrote the Worlds' Finest series, which was initially drawn by George Pérez and Kevin Maguire. Levitz and Keith Giffen collaborated on the Legion of Super-Heroes issues #17 and 18 in 2013. Levitz wrote a biography of comics creator Will Eisner which was scheduled for release in 2014. He joined the board of directors of Boom! Studios in February 2014.
Levitz received an Inkpot Award at the San Diego Comic-Con International in 2002 and the "Dick Giordano Hero Initiative Humanitarian of the Year Award" in September 2013 at the Baltimore Comic-Con.
Levitz is a Mac computer user.
Levitz has three children: Nicole, a public health executive; Philip, a lawyer; and Garret, who works in the entertainment industry.
Levitz has named the run of All-Star Comics featuring the Justice Society of America as his favorite. He names Roger Zelazny as his favorite science fiction writer, J. R. R. Tolkien as his favorite fantasy writer, David McCullough as his favorite history writer and Agatha Christie as his favorite mystery writer.