Captain Mar-Vell was a member of the Kree, the alien humanoid race which has built an empire throughout the galaxy known as the Greater Magellanic Cloud. More specifically, he was one of the so-called "White Kree," a Kree race which has the same color skin as Caucasian Earth humans, which is the result of interbreeding of many of the original blue Kree race with humanoids of other worlds, and which now far outnumbers the blue Kree, who, however, still dominate positions of wealth and governmental power in the Kree Empire.
The Supreme Intelligence, the fusion of great blue Kree minds of the past that governs the Kree Empire, had encouraged interbreeding with other humanoid races because it recognized that the blue Kree race had reached an evolutionary dead end, and hence the Supreme Intelligence itself, since it was composed of blue Kree minds, would also evolve no further. The Supreme Intelligence had hoped that interbreeding would infuse new vitality into the Kree gene pool, but in recent years it had come to believe that interbreeding was not having the desired result.
Then the Earth-born Fantastic Four's defeats of a Kree Sentry and of Ronan the Accuser drew the Supreme Intelligence's attention to Earth. Thousands of years before, the Kree had learned of the genetic potential of Earth's human beings for evolving superhuman abilities, and had even conducted genetic experiments on Earth which had resulted in the evolution of the Inhumans. Now, the Supreme Intelligence realized, superhumans such as the Fantastic Four were evolving on Earth spontaneously, and Earth human beings had the potential of evolving even nearly limitless psionic powers.
The Supreme Intelligence formed a complicated plan. It would take an Earth human being and rearrange his genetic structure so as to awaken in him the vast psionic power that Earth human beings could eventually evolve. Because the Supreme intelligence could not absorb an Earth human being's mind into itself directly, it would cause this human being's mind to become linked with that of a white Kree, since white Kree are biologically more similar to Earth human beings than blue Kree are. The Supreme Intelligence would then forge a psionic link with these two minds, aided by an unusual plant called the Millennia Bloom. The result would be that the Supreme Intelligence would immediately evolve almost limitless psionic powers. Then, it would use the Earth human being and the member of the white Kree, who would now both be mindless and fully under the Supreme Intelligence's control, to destroy the Earth, so that no Earth human being could ever arise with power matching its own.
Other factors entered into the Supreme Intelligence's calculations. It had telepathically become aware that the Kree Imperial Minister, Zarek, and Ronan were plotting its overthrow. After Ronan's defeat on Earth, the Supreme Intelligence had ordered that a Kree starship put the planet under observation. Zarek and Ronan planned to arrange for a Kree hero to be assigned to the mission, and to manipulate events so that he would then turn traitor. They would then frame him for creating a menace to the planet Kree-Lar, administrative center of the Kree Empire. Ronan would then kill the Supposed traitor, halting the menace. The Supreme Intelligence would be blamed for the near disaster, and Zarek and Ronan would supplant it as ruler of the Empire. The Supreme Intelligence intended to thwart this conspiracy, but also to make the Kree pawn that the plotters selected be the one whom he would have linked to the Earth human being.
The Supreme Intelligence searched for the white Kree who would best fit these many requirements, and found him in Captain Mar-Vell, who was, despite his youth, an acclaimed hero in the Empire for his victories in battle with the Skrulls and others. The Supreme Intelligence knew that Mar-Vell and his lover Una, a nurse in the space fleet, both possessed humane attitudes and sympathies, which had been suppressed in most other Kree in their education. Moreover, the Supreme Intelligence learned that a Commander Yon-Rogg lusted after Una, and was ruthless enough to use any means of disposing of his rival for her love. Through secret use of its telepathic powers, the Supreme Intelligence arranged for Zarek and Ronan to learn of all this. The two conspirators then named Mar-Vell to be the Kree spy on Earth, Yon-Rogg to be his commander, and Una to be a nurse on their starship.
After arriving on Earth, Mar-Vell witnessed the death of Dr. Walter Lawson, a scientist on his way to his new assignment at Cape Canaveral, Florida, in an auto accident. Coincidentally, Lawson bore an astonishing resemblance to Mar-Vell, so the latter decided to pose as him, the better to study Earth humans' progress towards developing means of space travel. As Lawson, Mar-Vell met Cape security chief Carol Danvers. Meanwhile, Kree Sentry #459, which the Fantastic Four had defeated, had been brought to the Cape for study, and Yon-Rogg, hoping that it would destroy Mar-Vell, re-activated it from his starship. Mar-Vell publicly appeared in his Kree helmet and uniform to battle the Sentry, and succeeded in defeating it. Onlookers, hearing the Sentry call him by name, believed him to be a new Earth-born superhuman adventurer named "Captain Marvel" and acclaimed him as a hero.
Over the succeeding months, as Ronan, Zarek, and the Supreme Intelligence had planned, Mar-Vell's sympathy for the people of Earth grew increasingly stronger, and many times he surreptitiously subverted orders from Yon-Rogg and Ronan that would bring harm to Earth people. Finally Yon-Rogg caught Mar-Vell in direct disobedience of an order from the Empire's Supreme Council, and Ronan sentenced Mar-Vell to death by firing squad. Mar-Vell was about to be executed on Earth when Yon-Rogg's firing squad was attacked by a contingent of Aakon warriors. In the melee Una was fatally wounded by a stray shot by an Aakon. Mar-Vell escaped on an Earth missile, and laid Una's corpse to rest on an asteroid. Yon-Rogg seized Mar- Vells missile with a tractor beam and hurled it into hyperspace. The Earth missile had no means of leaving hyperspace, and drifted there for 63 days, by which point Mar-Vell was in an utter state of exhaustion.
Ronan and Zarek brought the missile to a small world on the edge of the Kree Galaxy, and took the now unconscious Mar-Vell to a "multi-sensory illusion chamber" they had constructed. There they caused him to have the illusion that he encountered a godlike being named Zo which gave him the power that he wanted to be able to destroy Yon-Rogg. The powers that Mar-Vell then received were not illusory: he became far stronger than other known Kree (except for Ronan), and gained the ability to cast illusions and to teleport himself across even intergalactic distances. Mar-Vell briefly returned to Earth but Zo "appeared" to him again and ordered him to Hala to battle a gigantic generator of magnetic power, disguised as an idol of the Kree god Tam-Bor, which was capable of destroying the entire planet. Right after Mar-Vell found the generator, Ronan, carrying out his plan, appeared and accused Mar-Vell of being its creator. Mar-Vell and Ronan fought, but Mar-Vell succeeded in destroying the generator. The Supreme Intelligence's personal "Super-Sentry" then appeared and drove off Ronan, and brought Mar-Vell to the chamber of the Supreme Intelligence, where Zarek stood as a captive. Suddenly Ronan teleported into the chamber' and teleported out again with Zarek, leaving an antimatter bomb behind. Mar-Vell leapt upon the sphere to save the Supreme Intelligence, but the Supreme Intelligence explained that his devices had neutralized it. The Supreme Intelligence honored Mar-Vell with a special uniform, and allowed him to retain his newly increased strength and abilities to fly unprotected through space and hyperspace, although he could no longer teleport himself or cast illusions. However, as soon as Mar-Vell left for space, he found himself hurled into the Negative Zone, an antimatter universe in a dimension alongside that of Earth.
Next the Supreme Intelligence secretly enabled Mar-Vell to make telepathic contact with Rick Jones, a youth who had previously been a companion to the Hulk, Captain America, and the original Avengers. Mar-Vell's image lured Jones to a hidden and abandoned Kree base on Earth, where he had Jones put on a pair of "nega-bands" around his wrists, and then to slam them together. The result was that Mar-Vell and Jones traded places: Mar-Vell was now on Earth, and Jones, surrounded by a protective aura that allowed him to survive, was in the Negative Zone. As long as they wore the bands, Mar-Vell and Jones were in total telepathic contact; thus, the Supreme Intelligence had created the psychic link between an Earth human being and white Kree that he desired. The nega-bands remained on Earth on the wrists of whichever of the two-Mar-Vell or Jones--was on Earth at the time. Hence, Mar-Vell could return to the Negative Zone by slamming the bands together, thereby returning Jones to Earth. Mar-Vell could only remain on Earth for brief periods of time; if he stayed a total of three hours, he and Jones would switch places automatically. So it was Jones who spent most of the time on Earth, but he was willing, usually, to trade places with Mar-Vell at intervals.
Soon after becoming linked with Jones, Mar-Vell confronted Yon-Rogg, who had abducted Carol Danvers as a hostage to use against him. Mar-Vell defeated Yon-Rogg, who died when a nearby Kree device exploded. In the course of this battle Danvers was irradiated by a Kree Psyche-magnitron, causing her to develop a half-Kree physiology which was similar to Mar-Vell's; using the superhuman powers she gained. Danvers later became the first costumed adventurer known as Ms. Marvel. Sometime later, Mar-Vell used the Fantastic Four's machine that created a doorway into the Negative Zone to transport Jones out of the Zone, so that they both then existed in the Earth dimension simultaneously and separately. However, Ronan, who had been imprisoned along with Zarek, now broke free and seized control of the Kree Imperial government. Almost immediately afterward, war broke out between the Kree and Skrull Empires. Eventually, the Supreme Intelligence contacted Rick Jones and activated Jones's latent psionic powers. Jones used them to paralyze the entire Skrull space fleet as well as Ronan and his forces, thereby ending the war and allowing the Supreme Intelligence to return to power. The Supreme Intelligence then deactivated Jones's power, and the youth was left exhausted and lying on the point of death. As the Supreme Intelligence had planned, Mar-Vell again linked himself to Jones, in the same manner as before, so that his own life force would restore Jones to health.
Mar-Vell returned to Earth, where he learned that Jones's body could no longer contain both his own life force and Mar-Vell's. Jones was bombarded with a form of electromagnetic radiation by Professor Benjamin Savannah, which had two effects. First, Jones could now contain both his and Mar-Vells life forces. Second, Mar-Vell himself could now absorb solar energy and use it for strength and for firing bolts of force.
Soon afterwards, Mar-Vell became embroiled in battling Thanos and his plans for destroying all life in the universe. Mar-Vell was teleported to the presence of the extraterrestrial being called Eon which had been created thousands of years before to watch over the evolution of superhuman beings on Earth. Eon induced Mar-Vell to reject the Kree's love of warfare entirely, and to reach inner peace. Eon then granted Mar-Vell a psionic ability called "cosmic awareness", and designated him as "Protector of the Universe". Thanks to these changes in himself, Captain Mar-Vell became the primary force in bringing about Thanos's defeat.
Next Mar-Vell battled Nitro, who had stolen a powerful nerve gas. A leak developed in a canister of the gas, but Mar-Vell used his great strength to seal it. The gas caused Mar-Vell to collapse, but he seemed to recover fully afterwards. However, the gas was also carcinogenic, as he would learn later.
As time passed, Mar-Vell and Jones learned that they could both exist in the Earth dimension simultaneously by concentrating on the nega-bands. However, to maximize Mar-Vell's power when necessary, they would still merge. After Mar-Vell passed too closely to a black hole, Jones was again trapped in the Negative Zone, but Mar-Vell released him during a battle with the Super Adaptold. Mar-Vell and Jones had various other adventures, and finally, in confrontations with the Supreme Intelligence, learned its true plans for them, which Mar-Vell succeeded in thwarting.
Finally, however, the cancer that the gas had given Mar-Vell took its toll. Mar-Vell spent his last days on Titan where he died, surrounded by his lover Elysius of Titan, Jones, the Avengers, and various other friends.