Member of the Green Lantern Corps and the Justice League
John was originally a US Marine before becoming a Green Lantern. After leaving the service, John was chosen by the Guardians of The Universe to join The Green Lantern Corps There he was trained by veteran Lanterns Katma Tui and Kilowog. In his comic background he was an architect and was chosen to be the backup Lantern for Hal Jordan.
John was a founding member of the League. A former Marine, he was the "tough guy" on the team and would put his skills he learned from his days in the Corps to use when his power ring ran out of charge when he and the League were in World War II trying to stop time traveling warlord Vandal Savage. Also during his time in the League, he, Batman, and Wonder Woman would travel to a semi-alternate future to find the criminal Chronos. He would meet his and Shayera Hol's son, War Hawk who was part of the Justice League Unlimited along with Terry McGinnis and Static.
The Green Lantern
The original pitch for a Green Lantern film was to have trilogy of films focusing on John Stewart who was to be played by Cory Reynolds. The films would have introduced The Justice League and Hal Jordan would have been featured in a spin-off film.
Justice League Mortal
In the shelved Justice League film, John was set to be played by rapper Common.
In the Comics
John Stewart of the comics.
John Stewart in the comics differs from his animated portrayal greatly.The comics John Stewart had never really been a big player in the DCU until after Justice League came on the air. Also unlike the show is that John never had a relationship with Shayera Hol, as Shayera Hol was the Silver age Hawkwoman and wasn't around in the DCU when John was created. Though the comics version has taken influences, such as John becoming a Marine from the animated version. One of the biggest differences between the two is huis rivalry with Sinestro, while in the comics Sinestro hates all Green Lanterns, he's more of a foe of Hal Jordan rather than John Stewart.