John Stewart was an architect who was selected by the Guardians of the Universe as Hal Jordan's backup after Guy Gardner was seriously injured in a disaster. Although, Jordan objected after seeing that Stewart had a belligerant attitude to authority figures, the Guardians stood by their selection. Jordan complied and recruited and equipped Stewart with the standard uniform and power ring.
To Jordan, Stewart's first mission began badly with the assignment of protecting a racist politician and Stewart took advantage of averting an accident to embarass him in the process. However, Stewart soon proved his worth when an apparent African American assassin shot at the politician, but Stewart refused to intervene with Jordan to move in response to the attack. However, Stewart had good reasons for this apparent derliction of duty when he stopped a gunman from killing a police officer in the outside parking lot at the event while Jordan was pursuing a decoy. When Jordan confronted Stewart for his actions, Stewart effectively explained the situation of the ruse and the fact the politician staged it for political advantage. With that adventure, Jordan concluded that Stewart was an excellent recruit after all.
For some time, Stewart occasionally filled in for him as Green Lantern when Jordan was unavailable, including some missions of the Justice League. After Jordan gave up being Green Lantern, the Guardians selected Stewart to full time duty. Stewart filled that role for some years, during which time he married Katma Tui, the Green Lantern of the planet Korugar. After Tui�s murder, Stewart became the administrator of the �Mosaic World,� a patchwork of communities from multiple planets that had been brought to Oa by an insane Guardian. From this position, Stewart eventually ascended to Guardianhood, which he later relinquished. After a brief period of paraplegia, he became Green Lantern once again and became a member of the JLA. Stewart was also a member of the Darkstars for some time. He regained his power ring during Kyle Rayner's tenure as Earth's main Green Lantern, and remained his backup for years, until Hal Jordan was resurrected after his spirit was freed from Parallax and the Spectre, after which Jordan took over Rayner's role as Rayner went to serve an administrative position in the Green Lantern Corps in Oa.
In the subsequent restructuring of the Corps, each sector now has two Green Lanterns assigned to it, and Stewart and Jordan now share the responsibilities for Earth's sector 2814. After the dissolution of the Justice League in the aftermath of the Infinite Crisis event, and the destruction of their Moon Watchtower headquarters, Stewart has begun playing a larger role in metahuman affairs, working with many former Justice Leaguers.