In this modern age of comic-influenced pop culture we’re neck deep in shared universes and, increasingly, shared multiverses. It can be hard to imagine a time when that wasn’t the case. It can be even more difficult to believe we didn’t see the first true comic book shared universe until the mid-1960s. That’s when the daughter of the third most important character to debut in Action Comics #1 arrived on the scene in a quest to find her father. Zatanna’s search would take her on a winding path through six DC titles. As a result, it impacted several heroes and had repercussions on the DC Universe as a whole.
Characters with their own titles crossing over with other characters is a common occurrence going all the way back to the birth of superhero titles. It’s difficult to pinpoint who can lay claim to the first ever superhero crossover.
The honor could possibly go to Lev Gleason Publications in 1940. That publisher teamed up the characters Silver Streak and Daredevil (not that Daredevil). It just barely preceded the first known team up of Marvel characters. Marvel Mystery Comics #8 featured Namor and Human Torch going head-to-head for the first time. For DC characters, the first crossover medal goes to All-Star Comics #3 which saw the first appearance of the Justice Society of America. At the time, it was likely the biggest crossover in terms of number of characters featuring Flash (Jay Garrick), Green Lantern (Alan Scott), Hawkman (Carter Hall), Spectre (Jim Corrigan), Hourman (Rex Tyler), Sandman (Wesley Dodds), Doctor Fate, and Atom (Al Pratt).