The 20th century was a turning point in the history of Earth. The population explosion following World War II caused an increase in the crime wave. To counterbalance this, many humans took up vigilantism or used scientific advancements to fight crime. By the end of the century, mankind had begun working on splicing the genome, gaining advances in nanotechnology, sending men to Mars and had sent small ships out to Alpha Centauri. Earth had just begun contact with the galactic community.
- Cyrus Gold is killed and his body is dumped into a mystical swamp. He begins the process of transforming into Solomon Grundy. (JL: The Terror Beyond)
- World War II begins on Earth. Soon after, Vandal Savage receives information from his future self that he uses to take over Europe. (JL: The Savage Time)
- The Justice League arrives from the Modern Age, and fights against Savage's forces, with the help of Sgt. Rock, Easy Company, the Blackhawks and Steve Trevor. (JL: The Savage Time)
- World War II ends on Earth.
- Aresia is brought to Themyscira by a sea captain after a battle and a shipwreck. (JL: Fury)
- Solomon Grundy rises from the dead, and becomes an urban legend among the populace. (JL: The Terror Beyond)
- The last Justice Guild comic book is released parallel to the deaths of the Justice Guild in a parallel universe. (JL: Legends).
- Jax-Ur and Mala are imprisoned in the Phantom Zone for instigating an uprising on the planet Krypton.
- Brainiac collects all of Krypton's knowledge before it is destroyed. He then leaves to collect all the knowledge in the galaxy. (STAS: The Last Son of Krypton)
- Kal-El's craft lands in Smallville and he is adopted by Jonathan and Martha Kent. He is given the name Clark Kent. (STAS: The Last Son of Krypton)
- Soul Power and Sparky protect the city of Dakota from crime using electricity-based powers. Their main enemy is Professor Menace. (SS: Blast From the Past)
- Thomas and Martha Wayne are gunned down in Crime Alley by an unknown gangster. This event is witnessed by young Bruce Wayne, who is taken in by former British Intelligence officer Alfred Pennyworth and family friend Doctor Leslie Thompkins. Bruce Wayne is eight years old. (BTAS: Appointment in Crime Alley)
- Bruce Wayne travels abroad to learn to be a crime-fighter. He begins by studying to be a detective, and he also studies science under Nobel laureates. (BTAS: Night of the Ninja)
- Bruce Wayne spends a month under magician Zatara's tutelage. While there, he learns escape artistry, and he develops an attraction to Zatara's daughter Zatanna. He goes by the name John Smith. (BTAS: Zatanna)
- Clark Kent meets the Legion of Super-Heroes when they attempt to ward off a future version of Brainiac. He ends up having his memories erased. (TNSA: New Kids in Town)
- Bruce Wayne trains under Yoru-Sensei. He meets Kyodai Ken, who is able to best him in hand-to-hand combat. (BTAS: Night of the Ninja)
- After many years abroad, Bruce Wayne returns to Gotham City. While attending college, he meets and gets engaged to Andrea Beaumont, daughter of Carl Beaumont. When Carl's mobster connections force them to move, Andrea breaks her engagement to Bruce. Around the same time, Bruce, still haunted by his parents' deaths, begins crime-fighting. At first, he attempts to do it dressed in common clothes, but then realizes that he must strike fear into the heart of his enemies and dons cape and cowl to become Batman. (Batman: Mask of the Phantasm)
- Batman's first Batmobile breaks down and he hires Earl Cooper, a former auto executive, to build him a brand new Batmobile. (BTAS: The Mechanic)
- Dick Grayson is placed into Bruce Wayne's custody after his parents are killed by Tony Zucco. (BTAS: Robin's Reckoning)
- Virgil Hawkins is born to Robert and Jean Hawkins in Dakota. (SS: Flashback)
- John Stewart is recruited by Abin Sur into the Green Lantern Corps. He begins his training with Katma Tui. (JL: Hearts and Minds)
- Carl Beaumont is murdered by a hitman hired by Sal Valestra. (Batman: Mask of the Phantasm)
- During a fight between Batman and Sal Velestra's goons, a mobster is dropped into a vat of acid, and becomes the Joker. (TNBA: Mad Love)
- Dr. Victor Fries is transformed into Mr. Freeze by Ferris Boyle and his security guards, while Fries attempts to cure his wife Nora of a disease. (BTAS: Heart of Ice)
- Batman fights a gang of jewel thieves. (The Dark Knight's First Night)
- The Gotham Tricentennial is celebrated to commemorate Gotham City's 300th anniversary. (TNBA: Cold Comfort)