He was a comic book and pulp magazine editor, and a science fiction agent and prominent fan. 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, one of the companies that evolved into DC Comics.

One of the most prolific writers in comics, particularly in the Silver Age. He took over scripting duties on Wonder Woman after William Moulton Marston's death, and handled the character's transition from the Golden to the Silver Age. He also created Barry Allen, the second Flash, for editor Julius Schwartz's superhero revival of 1956, as well as writing and editing DC's pioneering war titles. His creations include Sgt. Rock, the Unknown Soldier, Barry Allen, Ragman, the Losers, Black Canary, the Metal Men, Poison Ivy, Enemy Ace, the Suicide Squad, and Rex the Wonder Dog.