John Stewart was a Marine before becoming a Green Lantern in Lanterns.
The series presents Stewart as both a former Marine and an artistic architect. This combination gives the character two distinct dimensions: military experience that supports his discipline and physical ability, and an artistic background that reflects imagination and design.
Stewart is introduced as a new recruit being trained by Hal Jordan. His path into the Corps is unusual because he is described as the first Green Lantern recruited by the Guardians of the Universe rather than selected by a power ring, the normal process in the series’ mythology.
The profession is sometimes confused with the occupations of other characters. Hal Jordan is a former test pilot, Guy Gardner used to be a gym teacher, and the earlier version of the long-developed Green Lantern project had included an FBI agent named Alan Scott. Those details belong to different characters or earlier concepts, not Stewart’s current Lanterns biography.