40 years ago - Jon Stewart is born.
22 years ago - 18-year-old Jon joins the Marines.
14 years ago - 26-year-old Jon finishes with the Marines & begins working as an architect.
11 years ago - 29-year-old Jon is chosen as a back-up Green Lantern when both Hal Jordan & Guy Gardner are injured.
5 years ago - 35-year-old Jon is depowered when Hal Jordan destroys the Oan Power Battery, & returns to work as an architect.
1 years ago - 39-year-old Jon's ring is repowered when the Guardians restarts the Oan Power Battery, and takes Kyle Rayner's place in the Watchtower.
The prevailing opinion about Jon Stewart is that he was added to the Green Lantern simply because they wanted more diversity and couldn't be bothered to actually create a new character. He was black, he was angry, and he had a green lantern ring; that was the sum total of his character for some time. He was a minor background character in the story of Green Lantern, barely even showing up.
The main reason for his rise to prominence is the animated Justice League cartoon. With the show happening right in the middle of Kyle Rayner's time as the sole Lantern of earth, the debate as to whether to use Kyle or Hal was surprisingly heated, and the solution was to use neither of them. Essentially a blank slate, they used Jon as a veteran superhero with military training, someone that brought knowledge and experience to their team. Coupled with a soft-spoken tough-guy attitude and the immeasurable utility of his ring, he quickly became a fan favorite, and shortly afterward this characterization was taken up by the comics as well. It was so well recieved that a few years later, when the Green Lantern corp was restarted and the idea was introduced that every sector would be patrolled by a pair of Lantern partners, Jon is the only one of the numerous Earthborn Lanterns to have always help that job.
When reading the comics, Jon (as we now know him, anyway) doesn't really arrive as a viable character until after the fall of Oa, so we've made sure to give him a more prominent role in the years that precede it. He's still a back-up Lantern, so he isn't featured TOO heavily, just enough to carry his characterization through his entire career.