In 1932, Schwartz co-published (with Mort Weisinger and Forrest J. Ackerman) Time Traveller, one of the first science fiction fanzines. Schwartz and Weisinger also founded the Solar Sales Service literary agency (1934-1944) where Schwartz represented such writers as Alfred Bester, Stanley G. Weinbaum, Robert Bloch, Ray Bradbury, and H. P. Lovecraft, including some of Bradbury's first published work and Lovecraft's last. In addition, Schwartz helped organize the first World Science Fiction Convention in 1939.
In 1944 he became an editor at All-American Comics (which later merged into DC Comics). He recruited Bester to contribute to the company's line of comic books. In the 1950s he oversaw the revival of superheroes such as the Flash, Green Lantern, Hawkman and the Atom, which led to the Silver Age of comic books. This revival has been cited as an inspiration for the transformation of Marvel Comics in the 1960s. The Schwartz-edited line of titles was regarded by many as being more creative and dynamic than other DC titles of the time, notably the Superman line edited by Mort Weisinger.
In the 1960s, Schwartz began editing the Batman titles, helping craft the "new look" Batman which indirectly led to the Batman television series. He also helped writer Dennis O'Neil and artist Neal Adams come to prominence at DC Comics.
From 1971 to 1985 Schwartz was the editor of the Superman titles, helping to modernize the settings of the books and move them away from "gimmick" stories to stories with more of a character-driven nature. This included an attempt to scale back Superman's powers while removing kryptonite as an overused plot device. This proved short-lived, with Schwartz bowing to pressure to restore both elements in the titles.
As an editor, Schwartz was heavily involved in the writing of the stories published in his magazines. He worked out the plot with the writer in story conferences. The writer would then break down the plot into a panel-by-panel continuity, and write the dialogue and captions. Schwartz would in turn polish the script, sometimes rewriting extensively.
Schwartz featured as a character in the Ambush Bug titles by Keith Giffen, which he also edited.
Schwartz retired from DC in 1986 after 42 years at the company, but continued to be active in comics and science fiction fandom until shortly before his death. As a coda to his career as a comic book editor, Schwartz edited seven DC science fiction graphic novels, adapted from classic science fiction works by Harlan Ellison, Robert Silverberg, Bradbury, and others. In 2000 he published his autobiography, Man of Two Worlds: My Life in Science Fiction and Comics, co-authored with Brian Thomsen.
His wife, Jean (who had been his secretary before they married), died in 1986 from emphysema, after 34 years of marriage. Schwartz's relationship with Jean had been particularly close, and he never remarried or dated following her death. Not many years later, Schwartz's stepdaughter Jeanne — Jean's daughter from a previous marriage — died from the same illness under similar circumstances.
He remained a "Goodwill Ambassador" for DC Comics and an Editor Emeritus up until his death. He was a popular guest at comic book conventions, often attending between ten and twelve conventions a year. Schwartz was so popular that he could often not get through a meal in convention hotel restaurants without being asked to sign autographs and answer comic book history questions from fans.
In 1998, Dragon*Con chairman Ed Kramer established the Julie Award, bestowed for universal achievement spanning multiple genres and selected each year by a panel of industry professionals. The inaugural recipient was science fiction and fantasy Grand Master Ray Bradbury. Additional awards, presented by Schwartz each year, included Forrest J. Ackerman, Yoshitaka Amano, Alice Cooper, Will Eisner, Harlan Ellison, Neil Gaiman, Carmine Infantino, Anne McCaffrey, Peter David and Jim Steranko.
Schwartz received a great deal of recognition over the course of his career, including the 1962 Alley Award for Best Editor, and the Shazam Award for Superior Achievement by an Individual in 1972 for bringing the Marvel Family back into print.
Schwartz died on February 8, 2004, at the age of 88, after being hospitalized for pneumonia. He was survived by his son-in-law, grandchildren and great-grandchildren, whom he encouraged to refer to him not as "Great-Grandpa" but as "Super-Grandpa."
After Julius Schwartz passed away, DC Comics celebrated his contributions to comics by publishing eight stand-alone special issues of DC Comics Presents inspired by eight classic Silver Age covers. Each issue featured two stories based on a classic DC Comics cover of the past, reflecting Schwartz's frequent practice of commissioning a cover concept, then telling the writers to create a story around that cover. Schwartz or a doppelganger thereof appeared in all eight issues, serving various roles in the stories.