John Stewart left home to enlist in the United States Marine Corps at a fairly young age. After leaving the service, John Stewart was selected by the Guardians of the Universe to join the Green Lantern Corps. Early in his Corps career, John would train under the veteran Green Lantern Katma Tui.

As Sector 2814 was already under the protection of Green Lantern Abin Sur, John was eventually reassigned to another space sector. It was only several years later, following Abin Sur's demise at the hands of Sinestro that John would return to his home sector. Abin Sur's designated successor, Kyle Rayner, would himself require training before being assigned a sector of his own. John Stewart was therefore sent to stand guard over Sector 2814, while Kyle was sent to Katma Tui for training.

John became a founding member of the Justice League after teaming with other heroes to repel an alien invasion. As the group dynamic developed, John became particularly close with the Flash and Hawkgirl, Shayera Hol. His relationship with Shayera became romantic until Shayera was exposed as a Thanagarian spy and John faced off against her former lover, Hro Talak. After the League defeated the Thanagarins, they expanded the League by recruiting dozens of heroes to join them.

When the time traveler Chronos came back in time to steal Batman's utility belt, John, Wonder Woman and Batman chased him, first to the Old West and then to the future, where he met some members of the future justice league, Batman II, Static and Warhawk. While there, John discovered his future, He would become the father of Warhawk, and the mother would be none other than Shayera Hol. Once Chronos was defeated, everyone returned to their own time.

As one of the founding members of the League, John served as a voice of leadership within the organization throughout some of its most trying times, including the Cadmus crisis and the ongoing conflict with Grodd's Secret Society.