J'onn J'onzz was the last Martian and a founding member of the Justice League.
In the Justice League animated series, J'onn reached out telepathically to several heroes on Earth: Superman, John Stewart, Hawkgirl, Wonder Woman, and Flash after the Imperium began their invasion of Earth.
This is J'onn J'onzz' in his natural Martian form. J'onn had succeeded in releasing the nerve gas that placed the Imperium in suspended animation after they had invaded Mars. As the last surviving member of the strike team, and of the entire Martian race, J'onn then sealed the invaders' underground headquarters to keep them paralyzed, and stood guard over it for 500 years.
While J'onn was in a hibernation cycle, astronauts from Earth accidentally revived the invaders who now targeted Earth instead.
Banding together, the newly-formed team managed to destroy the Imperium. Mindful of future threats, Superman proposed the formation of the Justice League.