When developing the show, Bruce Timm toyed with the idea of giving the Justice League a uniformed look to their costumes, and giving Wonder Woman a new hair style. When Timm mentioned it to DC President Paul Levitz, he said, "Not the big three (Superman, Wonder Woman, and Batman). You can do it to Green Lantern, you can do Flash, but not the big three." The designs were later used for the Justice Lords costumes.

The John Stewart Green Lantern was selected, rather than the better-known Lanterns, Hal Jordan and Kyle Rayner, not only because it would allow for an African-American member of the team, who occasionally appeared in the Justice League comic book, but the producers felt that the character's originally abrasive personality would have more dramatic potential.

According to producer Bruce Timm, this version of John Stewart has been a Green Lantern for so long that the green radiation energy of the ring has infected his bloodstream, so that's why his eyes glow green.

Was originally to be titled "Justice League of America", after the incredibly popular "Justice League of America" comic (revamped by Grant Morrison). However, "Justice League" was favored, to give the show a more universal feel.

The Green Martian characters, such as Martian Manhunter, are voiced by African-American actors and actresses, and the Thanagarian characters, such as Hawkgirl, are all voiced by Hispanic actors and actresses.

Unlike Super Friends (1973), not a single comic book character appears in every episode of this animated series.

Though a founding member of the Justice League in the comic books, Aquaman was left out of the cartoon series' line-up, in favor of Hawkgirl, thus providing another female character. Aquaman appears only as a guest star.

Although several actors reprised their voice roles from the shows Batman: The Animated Series (1992) and Superman: The Animated Series (1996), conspicuously absent is Tim Daly, who voiced Superman. He became unavailable after landing the lead role in the television series The Fugitive (2000). However, this is the fourth consecutive animated series starring Kevin Conroy as Bruce Wayne, a.k.a. Batman, following the aforementioned 1992 series, The New Batman Adventures (1997), and Batman Beyond (1999) (not including guest appearances on Static Shock (2000) and The Zeta Project (2001)).

Hawkgirl was never seen without her mask in the first forty-eight episodes. After the events of season two, episode twenty-four, "Justice League: Starcrossed (2004)", she changed her costume to a jumpsuit, and never wore a mask again.

Only George Newbern (Superman) and Mark Hamill (the Joker) delivered their lines standing up. All the rest of the cast sat down.

Michael Rosenbaum (The Flash) also played Lex Luthor in Smallville (2001).

Green Lantern and Hawkgirl started to have a romantic relationship grow as the series progressed. The producers decided to pair these two characters together, since both were no-nonsense warrior types, with Green Lantern having a military background, and Hawkgirl coming from a very war-like planet. This relationship was originated for the show, as the two characters never had a romantic relationship in the comics.

Though Batman helps to get the Watchtower built by using surplus finances and technology from Wayne Enterprises, he is actually only a part-time member of the Justice League.

This is the first animated show to feature J'onn J'onzz, the Martian Manhunter.

To make this series less like Super Friends (1973), the producers decided not to use any sidekicks or junior members, or any animal mascots on the team. They were initially going to include Robin, Impulse, and Natasha Irons (Steel's little sister as a female Cyborg) as junior members when creating test footage for Kids' WB, but decided not to when Cartoon Network green-lit the show and did not request junior members.

The entire Marvel Family, and all related characters, were unable to appear on the show, due to legal reasons. In fact, the creative team had planned to use Captain Marvel in "Justice League: Hereafter (2003)". However, since they could not clear an appearance by Captain Marvel, they used Lobo instead.

This is the fourth time that Bruce Timm and his team redesigned Batman. This version is similar to the classic Bob Kane Batman, which also is similar in design to the Batman Beyond (1999) design. Similarities between the two are the lengths of the ears on the mask, the figure, and the boots having heels.

When developing the show, Bruce Timm thought it would be a good idea to have a CGI introduction sequence, because he liked how his character designs looked in 3-D after seeing game footage for Batman: Vengeance (2001).

Cheetah was supposed to have been killed by Solomon Grundy in her original appearance, but was shown to have survived, due to an animation mistake.

When the series began, Kyle Rayner was the active Green Lantern in the "Justice League of America" comic. After the animated series continued to gain in popularity, John Stewart was brought back to the comics in early 2003, and had replaced Rayner as the Green Lantern in the "Justice League of America" comic. Stewart was even redesigned to feature the fade haircut and uniform design from the animated version.

When developing the show, the producers had so many characters to use (with seven main characters - more than any of their previous DC Animated Universe shows) and they wanted to up the ante on the scale and the action, so they decided early on to do extended storylines, so every episode is one part of a two-parter, or even a three-parter.

The Justice League's headquarters is the Watchtower, which is from the modern mythos, but it's also the Justice League Satellite from the Silver Age. The producers decided to use a combined version of the two headquarters, because they thought it just didn't make sense to put the team all the way out on the moon, where the Watchtower was located in Grant Morrison's "Justice League of America" comic. It's just a little bit too far away.

Beginning with the second season, episodes were filmed in widescreen. Any second season episode broadcast as full-screen actually has the sides "cropped".

In this series, the creative team decided to discard the futuristic look of Metropolis, seen in Superman: The Animated Series (1996), and redesigned it to look more modern and realistic.

The producers felt that the early Gardner Fox stories made the characters almost interchangeable, and the more recent Grant Morrison stuff was just way too dark and complicated. Therefore (with the exception of "Justice League: In Blackest Night (2001)", which was based on one of Fox's stories), they decided to create their own stories. However, what Bruce Timm did use from the Morrison run on "Justice League of America", was the "dysfunctional family" atmosphere for the League.

In designing Wonder Woman for the show, the creative team stuck mainly to the modern, Post-Crisis incarnation of the character that debuted in 1987. The world of Diana and the Amazons was written to be closer in tune with the original Greek and Roman myths, while, at the same time, incorporating elements from the previous versions of the Wonder Woman mythos. However, her origin was adapted to fit in with the events of the White Martian invasion, meaning that elements such as the contest to decide which Amazon would become Wonder Woman, and Diana's sanctioned mission as Ambassador to Patriarch's World were eliminated.

To make the show unique, the producers decided not to use the established Superman and Batman villains that they had used in the past on other shows. They figured this show presented a really good opportunity to choose from a wealth of DC villains, such as Vandal Savage, Dr. Destiny, Felix Faust, and Gorilla Grodd. The only established Superman or Batman villains they planned on using were Lex Luthor and the Joker. Both were in the Injustice Gang. They wanted to try something different for as long as they could, before going back to using established Batman and Superman villains in season two.

Since Batman: The Animated Series (1992) and Batman Beyond (1999) all went out with a "poof", the creative team planned on ending each season with a "bang", as if it was their last, just in case. The producers decided they wanted to have a satisfying conclusion to each season, but still keep the door open to continue the series should it get picked up for another season.

When developing the show, the producers learned from Marvel's mistake on Avengers: United They Stand (1999) by not including Captain America, Iron Man, and Thor. This show always intended to include Superman, Batman, and Wonder Woman. According to Bruce Timm, DC Comics President Paul Levitz was behind them all the way, saying right off the bat, "It's going to be hard to get the rights to all of these characters, but you have to have Superman, Batman, and Wonder Woman, and whatever it takes, we'll make that happen."

The character models for some of the characters in the test footage were quite different from the final product. The Superman design used was identical to the models used on Superman: The Animated Series (1996). Green Lantern had the same hairstyle and costume as the final design, but still possesses the dark green mask from the Superman draft model. The one with the most significant changes to his design was J'onn J'onzz. In the test footage he appears blockier, with a squarer jaw, a Kirbyesque line detailing his right cheek, and black eyes with red pupils (similar to the Joker's redesign for The New Batman Adventures (1997)). In addition, J'onn's color palate utilizes paler greens, blues and reds, and possesses none of the shiny highlights that have become a trademark of the series. Featured briefly commanding a robot that resembles the Golem (from Batman Beyond: Golem (1999)), similarly, Lex Luthor had a craggy, Kirby-like detailing on his face, and his design is an early take on the updated purple-and-green jumpsuit uniform, indicating that the creative team was already planning on utilizing the Silver Age "mad scientist" Luthor, rather than the "corrupt industrialist" Luthor, from Superman: The Animated Series (1996). Solomon Grundy looked a bit younger than on the final product and Cheetah looked more like her comic counterpart, featuring long black hair.

At the request of DC Comics, the red of The Flash's costume was a darker shade than it was on his initial appearance on Superman: The Animated Series (1996). Also, the circle around the Flash symbol on his chest was changed from black to yellow.

Producer Bruce Timm had said it's great to be on Cartoon Network, because the creative team got very little Broadcast Standards & Practices notes from them.

Green Lantern's characterization differed from the comic book version of John Stewart, by being a former United States Marine, and having not been explicitly revealed to have studied architecture. Since then, the comic book version has been updated to include being a former Marine.

Glen Murakami and James Tucker thought they needed a new Superman design for the show, thinking he should be a little more mature, a little more rugged. Bruce Timm pushed to just use the old Superman model from Superman: The Animated Series (1996), but eventually went along with the idea, and gave him a bulkier physical appearance, with strong cheekbones, and little tiny dashes under his eyes, which are supposed to give him a little bit more of a comic book "squinty" look, making him look a little bit older. Starting in season two, the strong cheekbones and dashes under the eyes were removed.

The Justice League uses their ultra-advanced spacecraft. The reason the producers gave them a spacecraft is that the Watchtower is out in space, so they needed a way to get to and from it, but they didn't want to do transporters (though the team did use them in the comics for some time).

When developing the show, the producers intended on using The Key, but scrapped plans to use him, because his powers were a little too close to Dr. Destiny's. The Key was nearly omitted from the DC Animated Universe, until he made his long-awaited debut as a member of Grodd's expanded Secret Society in Justice League Unlimited (2004). In addition, the creative team updated The Key's look, giving him a Jack Kirby-inspired update of his classic Silver Age costume.

The show did a two-part crossover with Static Shock (2000). The "Static Shock" crossover was originally going to be with Teen Titans (2003), but since the crossover would have aired before the "Teen Titans" animated series premiered, a crossover with the Justice League was done instead.

Keith David not only does the voice-over work on the Cartoon Network commercials for the show, he is also the voice actor for Despero.

For season two, episode twenty-four, "Justice League: Starcrossed (2004)", the creative team wanted to use Hawkman, but was denied by DC Comics, since the character acted more like a villain in that story. The character was renamed Hro Talak (an anagram of Katar Hol) and though loosely based on the Silver Age Hawkman, he wasn't allowed to be Hawkman.

Though intended to incorporate both the previously aired Batman: The Animated Series (1992) and Superman: The Animated Series (1996) cartoon histories, the creative staff initially decided to ignore "Superman: The Animated Series: In Brightest Day... (1999)" so as to not confuse viewers with two different Green Lanterns. This plan was later reconsidered. In "Justice League: Hearts and Minds (2003)", the Green Lantern introduced in the "Superman" series, Kyle Rayner, is briefly mentioned as being a Green Lantern Corps member, and appears in "Justice League: Hereafter (2003)". In "Justice League Unlimited: The Return (2004)", Rayner is shown in action with the Corps as a speaking part. In "Justice League Unlimited: The Once and Future Thing, Part Two: Time, Warped (2005)", Chronos's tinkering with space-time causes John Stewart to briefly morph into Hal Jordan, the original comic's Silver Age Green Lantern, who was not previously seen in Batman: The Animated Series (1992), Superman: The Animated Series (1996), or The New Batman Adventures (1997).

The show had a spin-off comic book, called "Justice League Adventures", which ran for thirty-four issues from 2002-2004.

After using Aquaman's classic design on Superman: The Animated Series (1996), the producers warmed up to his redesigned look from the comics, with the long hair and beard, with hook hand, and decided to use it for the show.

Aresia is based two individual characters who use the identity of Fury in the comics. This animated version is based more upon the Helena Kosmatos version, but possesses a visual design based upon the original Fury.

For this series, AMAZO got a complete redesign. Producer Bruce Timm thought AMAZO's design in the comics was really old-school, real garish and bizarre, and not very modern at all. Therefore, he came up with a new design that was a combination between Gort from The Day the Earth Stood Still (1951), with the morphing powers of the T-1000 from Terminator 2: Judgment Day (1991). His creator, Professor Ivo, a classic Justice League villain, never makes an appearance in the animated series, and it is unknown to what end the android was created.

During this series, Green Arrow was the most requested character by fans. However, he never made an appearance in the DC Animated Universe, until Justice League Unlimited (2004).

The animated Thanagarians appear to have wings as part of their bodies.

Originally, the creative team wanted to feature the old Justice Society in "Justice League: Legends (2002)", but DC had some issues with their use in this particular story, so they compromised and made the Justice Guild of America, who are inspired by the Golden Age Justice Society.

This version of Cheetah is the post-Crisis Barbara Ann Minerva version, but this version has a different background, and does not have the long hair like her comic book counterpart.

Some villains on the show, like Ocean Master and Eclipso, are never referred to by their comic book codenames.

The Superman, Batman, Hawkgirl, and Green Lantern Justice Lords costumes are alternate looks of the future costume changes of the regular heroes: Superman changes his costume to a black and white Kryptonian costume, and gets attacked by Starro; Batman changes to the Batman Beyond (1999) costume, with the same shaped bat symbol; Hawkgirl uses a Thanagarian-inspired mask in "Justice League: Starcrossed (2004)"; and Green Lantern shaves his head, following the events of "Starcrossed".

The show's creative team, even going back to Superman: The Animated Series (1996), had no intention of using Doomsday. They only decided to use him to show how ruthless the Justice Lord Superman is.

After appearing in the recent Grant Morrison "Justice League of America" comic, the Injustice Gang made their animated debut on this series (in "Justice League: Injustice for All (2002)"), and the team consisted of Lex Luthor, the Ultra-Humanite, Copperhead, Star Sapphire, Shade, Cheetah, Solomon Grundy, and The Joker. They appeared again in "Justice League: Fury (2002)", minus Lex Luthor, Joker, Cheetah, and the Ultra-Humanite, but with Aresia and Tsukuri added to the team. Since most television viewers were familiar with the Legion of Doom, the team was never referred to on-screen as the "Injustice Gang", until the Shade made a reference to his old team to the Secret Society.

The creative team chose to pay homage to both incarnations of Despero, combining the pre-Crisis mental powers and the throne of Kalanor with the post-Crisis hatred and the more physically imposing body type, to create an amalgam possessing the most memorable traits of both. They also altered his skin tone from pinkish-purple to straight purple, and an gave him an eloquent, poetic slant to his speech pattern.

The Crime Syndicate was initially going to be used in this series, but as the writers continued to develop the story, the more they realized they were changing the basic idea of the Crime Syndicate so much that it wouldn't be worth calling them that. The team ended up being an alternate universe version of the Justice League, called the "Justice Lords". The real Crime Syndicate never appeared in the series.

There was a "Justice League" direct-to-video movie in the works, called "Justice League Worlds Collide", featuring the Crime Syndicate, and taking place between season two and season three. What happened was, the home video division commissioned a "Justice League" direct-to-video movie, right around the time work began for season three, so the original plan was to do the direct-to-video and season three concurrently. Dwayne McDuffie and Bruce Timm came up with the story, McDuffie wrote a terrific script, Timm designed loads of new characters, they started story-boarding, Andrea Romano had assembled a killer cast, and they were just getting ready to record it, and then home video slammed on the brakes, and it was put on the shelf for the time being. The season two finale was supposed to air in 2003, but was held back until May 2004, around the time the direct-to-video was supposed to be released. The project did see the light of day, heavily re-written as the non-DC Animated Universe Justice League: Crisis on Two Earths (2010).

StarToons, a Chicago-based animation studio which worked on many previous Warner Brothers shows, wanted to work on the series, and even submitted some test animation and storyboards, but were rejected by Sander Schwartz, who had recently become the President of Warner Brothers Animation.

The Flash in this series is Wally West, not Barry Allen. The producers didn't know which Flash they intended him to be under the mask. It wasn't until "Justice League: Starcrossed (2004)" that they finally settled on an identity.

During a meeting with DC Comics Vice President Paul Levitz and the show's creative team, Levitz suggested that somewhere in the course of the show, they should do a big stunt where one of the major characters gets killed, or one of them betrays the League. They quickly narrowed down the list of suspects to Hawkgirl, and thought it would be much more of a tragedy if she betrayed the League against her better judgment. If she was conflicted about it, and not just outright evil.

The original Watchtower is destroyed in the climax of "Justice League: Starcrossed: Part III (2004)".

"Justice League" incorporated many adult themes into some stories, like in "Justice League: A Better World (2003)", it is said that the Flash had been killed off-camera, and finally in "Justice League: Starcrossed: Part III (2004)", Green Lantern pushes the envelope by declaring to the Thanagarian warrior near the end, "Kiss my a**!". Cursing in the DCAU was then unheard of.

Voicing Flash in other DC animated projects, such as Justice League: Doom (2012), Michael Rosenbaum has voiced both Barry Allen as well as Wally West.

Carl Lumbly played M'yrnn J'onzz, J'onn J'onzz's father, in Supergirl (2015).