The Justice League formed when Earth faced some of its most terrible dangers, the top heroes of DC Comics working together to save the planet. At the time, the idea was to bring all DC's biggest and best heroes together in one book, although it was difficult because creative teams had control over what their characters did.
The first appearance of the Justice League came in The Brave and the Bold #27 by Gardner Fox, Mike Sekowsky, Bernard Sachs, Joe Giella, Murphy Anderson & Gaspar Saladino, where they battled Starro the Conquerer. However, this was not the first time the group teamed up, their true origin coming in the pages of Justice League of America #9 by Gardner Fox, Mike Sekowsky & Bernard Sachs.
Martian Manhunter
The first time the Justice League teamed up was to battle aliens from a planet known as Appellax. The storyline had different aliens battling for the rule of their home planet, with whoever could conquer Earth winning that role. The first man to face these aliens was Martian Manhunter, making him officially the first hero to battle as a founding member of the team. Manhunter didn't last as long as others, leaving the team in Justice League of America #71 and was gone from the JLA comics for 15 years.
Aquaman
The second person to show up in the Justice League origin story was Aquaman. Martian Manhunter was taken captive, and Aquaman was in the oceans when a fish arrived and told him that his fellow fish had been turned to glass by aliens. Aquaman tried to save the sea life but was also captured by the aliens. Aquaman ended up serving as a member throughout most of its history and even took over as the Detroit team leader.
Wonder Woman
The next member to join was Wonder Woman. She saw the aliens attacking Paradise Island and failed to stop them on her own. She showed up almost at the same time as Aquaman. While introduced as a powerful warrior in her own comics, Diana was actually listed as the secretary for the Justice League in her early days, a shocking title for a very different era. However, that was short-lived, and Wonder Woman became the heart and soul of DC's most powerful team.
Green Lantern
While Martian Manhunter, Aquaman, and Wonder Woman were battling to save the Earth from the invasion. Green Lantern was not on the planet. However, luckily for the other heroes, Green Lantern was on his way back to Earth and noticed a meteor headed toward the planet.
Much like the other heroes, Lantern was also stopped momentarily, thanks mostly to the aliens' yellow color. He ended up being captured alongside the other three Justice League founders.
Flash
Up next was Flash. This is interesting because when DC superheroes returned in the Silver Age, Flash was the first character to return in a new form, as Barry Allen was the first new hero created for the new generation. However, he was the fifth to show up when the Justice League formed. The good news is that the five heroes worked together and escaped, taking the battle back to the aliens.
Superman
The next two heroes arrived at the same time. The five superheroes showed up in Greenland, where they found both Superman and Batman battling an alien together. Martian Manhunter even humorously stated that these two heroes often work together. Superman is lucky that Batman was there to help them because the meteor Green Lantern saw was Kryptonite. Superman, a lifelong member, was never around much for the first missions of the team after this.
Batman
Much like Superman, Batman also wasn't around a lot after he joined the team. The two biggest heroes in DC Comics were often there as special guests, despite being full-fledged members, for the first few adventures. The truth is that the two heroes' editors felt that they would be overexposed if they appeared too much outside their own books.
However, Batman also has one other major spot in Justice League lore. After the seven heroes beat the aliens, they all gathered together, and it was Batman's idea to form the Justice League, calling it "a club or society," before Flash decided on "league."
Green Arrow
When the Justice League explained their origin story in Justice League of America #9, Green Arrow was there and was just interested in hearing the story. Arrow was the first new member to join the team after its formation. Green Arrow joined the team in the fourth issue and his admission to the Justice League when they invited him to join the team over other possible members like Adam Strange and Hawkman. It ended up as a unanimous vote of yes for Green Arrow to join the team.
Atom
The next new member came in Justice League of America #14 when Atom joined the team. The team opened the issue by voting on a new member, and the winner by unanimous decision was for The Atom. However, the Justice League members forgot who they were and what powers they held and forgot everything they knew about Atom thanks to the villain known as Mister Memory. After Atom helped saved the team, they made his membership official.
Hawkman
It was two years before the Justice League brought on another member. This took place in Justice League of America #31 when Hawkman joined the team. The issue opened with the Justice League message that Hawkman received membership for life. Atom was sent to meet with Hawkman and Hawkgirl, and he offered Hawkman membership since only one new member could join at a time. It took 13 years before Hawkgirl became a member.