Captain Mar-Vell of the Kree Imperial Militia had been sent to observe the planet Earth, which was of great interest to the Empire for its resource potential. Eventually, he realized the intentions of his superiors and rebelled, taking allegiance with Earth while the Kree Empire branded him a traitor. Thus now independent, the Captain fought to protect Earth from all menaces.
Later, he was punished with exile in the Negative Zone. The only way he could temporarily leave was to convince a young man, Rick Jones, to wear special metal wristbands called Nega-Bands which allowed the two to switch places for a limited amount of time, much like the situation with the original Captain Marvel. Together, the pair continued the battle against evil.
Eventually Mar-Vell was freed and then became an appointed cosmic champion of space in general. However, this career was cut short because of an earlier incident when he stopped a villain from stealing a canister of deadly nerve gas. The canister leaked, and, although Mar-Vell was able to re-seal the container, he had already been exposed to a lethal dose (due to his alien metabolism, he was not killed instantly, but rather contracted incurable cancer).
He died on Saturn's moon, Titan, in the presence of Marvel Universe's superhero community (as chronicled in Marvel's first regular graphic novel, The Death of Captain Marvel).