Thanos was one of the last sons of A'Lars, progenitor of the second colony of Eternals on Titan, and Sui-San, the last survivor of the original settlement of Eternals on Titan. Born with grey, hide-like skin and a massive body due to being born with the Deviant Syndrome, Thanos was a morose child who became obsessed with the concept of death.
Through bionic implementation and long hours of meditation, Thanos augmented his Eternal strength and powers so that his powers surpassed those of all other Titanian Eternals.
As he grew in power, Thanos also grew in ambition and desire for conquest. Thanos stole one of his people's starships and traveled to other star systems to recruit soldiers, mercenaries, and malcontents for a private army. With a small fleet, he dropped nuclear devices on his home world Titan, killing thousands of his people, including his mother Sui-San. Declaring himself ruler of Titan, he then set his sights on Earth.
At some point in his adult life, Thanos met the embodiment of Death itself who, in female form, became Thanos' companion. It is probable that Death was attracted to Thanos by both his philosophical dedication to nihilism as well as his willingness to commit genocide on even his own people. Thanos began, for the first time in his life, to experience love for another being, although the being was just a manifestation of Death. To make himself worthy of so awesome an entity, Thanos decided to acquire more power. At first he planned simply to seize political power through the conquest of worlds.
To this end, he amassed a huge armada of battleships. Then he found a way to amplify immeasurably his personal power. Having kept the planet Earth under surveillance since it first exploded a nuclear weapon, Thanos learned of a power-object called the Cosmic Cube brought into existence by the subversive organization, Advanced Idea Mechanics. Thanos seized control of the reality-altering device and commanded it to give him control over the universe.
Transformed into a godlike wraith by the Cosmic Cube, Thanos battled the Avengers and the Kree Captain Marvel, the former of whom had already vanquished his armada. When Thanos carelessly discarded the Cube, believing falsely that he had drained it of all power, Mar-Vell struck the cube, willing it to restore the universe to what it had been. Robbed of power, Thanos reverted to mortal form in the center of the universe where his flagship Sanctuary II retrieved him under pre-programmed instructions. Revived, Thanos was dejected to discover that Death had abandoned him because of his failure.
He soon formulated a plan to regain "her" affection: he would give to "her" more than any single living being had given "her," the annihilation of everything that lives. Through the minds and records of countless civilizations he searched, hoping to find the key to the power he sought. Eventually he learned of the Infinity Gems, six power objects of unknown origin and vast power, and set about to acquire them. He had obtained all of the Infinity Gems except one, the jade jewel worn on the head of Adam Warlock.
Thanos feared to acquire Warlock's gem directly because it possessed the power to steal souls, and he feared to risk forfeiting his own. Observing him, Thanos learned that Warlock had diverged a temporal counterpart named the Magus, who was the head of a galaxy wide religious empire and would certainly oppose Thanos' schemes of stellar destruction were he to learn of them. Thanos thus planned to eliminate the Magus. Possessing the capacity for time travel, Thanos decided to set up certain factors in the time stream which, combined, might prove the Magus' undoing. (He could have tried to thwart Warlock's life well before the critical time he would/wouldn't become the Magus, but he needed Warlock to remain the Soul Gem's possessor for simplicity's sake).
One of these factors was the rescuing of the infant Gamora, a female of the pacifistic Zen-Whoberis race that was slaughtered by the Magus' missionary troops. She would serve as Thanos' personal assassin. The second major factor would be Thanos' own direct alliance with Warlock against the Magus. Neither of these factors existed in the original time-line that led to the Magus' creation; thus, Thanos hoped to diverge a reality wherein Warlock never became the Magus. Thanos succeeded, and while in close proximity to Warlock, siphoned from his Soul Gem the energies he needed without Warlock's knowledge.
Thanos then constructed a single huge synthetic Soul Gem to hold the energies of the other six. With it, he planned to extinguish the stars one by one. Gamora learned of his plan of destruction and tried to assassinate him, but Thanos slew her. Thanos also slew Warlock's comrade Pip the Troll. Warlock had by this time learned of Thanos' threat and traveled to Earth to enlist the aid of the Avengers and Captain Marvel. In battle aboard Thanos' starship Sanctuary II, Captain Marvell damaged Thanos' projector through which he transmitted the synthetic Soul Gem's energies into suns to cause them to go nova. Warlock faced Thanos in personal combat and was slain. The Avengers were taken captive by Thanos, but rescued by Spider-Man and the Thing. Spider-Man, influenced by the abstract entities Lord Chaos and Master Order, released Warlock's spirit from the Soul Gem Warlock had worn, and seething with cosmic power released through his physical death, Warlock grappled with Thanos and turned the mad Titan to immobile stone. Incapacitated, Thanos retained a fragment of his consciousness and was thus tortured by the fact that he who worshiped death would forever be denied it.
However, after he obtained and lost the Infinity Gauntlet, he was led to a period of introspection. Adam Warlock recognized that Thanos' efforts to obtain omnipotence had been thwarted not only by the intervention of superheroes but by Thanos' own self-doubt and anxiety. Believing the Titan to be capable of change - and wishing to keep Thanos where he could keep an eye on him - Warlock made Thanos a member of the Infinity Watch, giving him the Reality Gem, one of the Infinity Gems which Adam Warlock and the Watch guarded.
Eventually, however, Thanos detected a threat to the universe, as the embodiment of time itself was rendered catatonic. Tracking the source to the revived Magus, Thanos sought the help of Adam Warlock and the Infinity Watch. Thanos and the Infinity Watch traveled to the realm of Death to learn the truth about the Magus, and found that he was attempting to replace this reality's heroes (and indeed, all reality itself) with evil doppelganger versions of his own. Returning to reality, Thanos and the Infinity Watch were attacked by an assemblage of heroes before the true threat of the Magus could be revealed. Taking the fight to the Magus, Warlock reassembled the Infinity Gauntlet in order to match the combined might of the five Cosmic Cubes possessed by the Magus, although ultimately Magus would gain the Infinity Gauntlet for himself. During the battle, Thanos was pitted against his doppelganger, who had become the Magus' most favored lackey, and in defeating it, gained insight into Magus' schemes. Confronting him, Thanos pointed out that Magus didn't, in fact, have the full power of the Infinity Gauntlet, as the Reality Gem was actually an elaborate fake. Rattled, the Magus was then thwarted by the embodiments of Eternity and Infinity, and the Infinity Gems were restored to their owners. Unknown to all, the Reality Gem was in fact held by Thanos himself, and had been since Warlock first distributed the Gems to the Infinity Watch. Thanos then returned to wandering the cosmos.
Some time later, Thanos and Warlock were trapped in the folds of a shifting reality, due to the appearance of the Goddess, which was Warlock's manifestation of his "good side" that had appropriated the power of the five Cosmic Cubes previously used by the Magus. Warlock eventually freed himself and Thanos. Thanos embarked on mission to learn about the Goddess and determine what threat she posed to reality. The two were soon contacted by the demon Mephisto, and Thanos agreed to let Mephisto aid them in exchange for a Cosmic Cube of his own. Warlock had a plan of his own, and before bodily entering the Soul Gem, he gave it to Thanos. Thanos, in turn, assembled a contingency of reality's heroes in order to distract the Goddess. Thanos however, seemingly failed, and the Goddess began to destroy the universe one star at a time. In reality, however, the destruction was an illusion generated by Warlock, who had used the Cosmic Cubes to create a universe-wide mass-hallucination. Distracted, the Goddess was defeated by Thanos and the assemblage of Earth's heroes. Thanos destroyed the "Cosmic Egg" that held the cubes, for which act Warlock tried to paint the Titan a hero. Thanos even honored his bargain with Mephisto, giving him a Cosmic Cube, albeit one without power.
Later, when the thunder god Thor apparently went insane, Thanos was contacted by the Silver Surfer to stop the mad god's rampage. Thanos succeeded where the Surfer, the Infinity Watch, Doctor Strange, and the other Asgardian gods could not. This led the assembled group to Asgard, where they came into conflict with Odin. Thanos battled Odin until the god learned the truth, revived Thor, and allowed Thanos and the heroes to leave Asgard with his and Thor's gratitude.
Soon after these events, Thanos became bored and decided to search for the vast storehouse of knowledge called the Oracle. To assist him, Thanos recruited several unwilling allies, which included the second Super-Skrull, Nitro, the Rhino, and the Titanium Man. After they accomplished his goals for him, Thanos stranded them.
After slaughtering the entire Ovin Mercenary Army just for something to do, Thanos used the Oracle to search for a worthy adversary. He believed he found one in Tyrant, first spawn of the world-devouring Galactus. With the help of the former Herald know as Terrax, he kidnapped Tyrant's archenemy and recent opponent, Ganymede. The hero Jack of Hearts, Ganymede's lover, recruited Genis-Vell to help free her. Ganymede, in the meantime, had joined Thanos and Terrax, in order to extract revenge on Tyrant. The trio battled Morg, Galactus' most deadly Herald and forced lackey of Tyrant, who eventually escaped. Jack of Hearts, Legacy, Ganymede and Terrax could not defeat the Tyrant. Thanos, who briefly fled the scene in order to learn the truth about Tyrant's origins, returned to battle Tyrant himself (with the help of one of Tyrant's own "power orbs" which contained the energies siphoned from Morg), laying waste to a sizable portion of Tyrant's planet. Thanos emerged from the battle declaring that, since he had withstood the powerful Tyrant for a time and gained what he sought from him, further struggle was pointless (and admitted it would likely have resulted in his own destruction).
The Silver Surfer, later, sought out Thanos in order to discern Death's motives for plaguing him with images of death with the obvious intent on making him her new consort. Enraged, Thanos beat the Surfer to death and delivered his body to Death's realm himself. Believing Death to be unappreciative, Thanos revived the Surfer and sought to leave, further insulting Death who cursed the Titan with immortality, forever barring him from her embrace.
Others would continually come in and out of Thanos' life. Gamora returned to Thanos' side, forsaking her relationship with Adam Warlock and the Infinity Watch. The computer intelligence know as Quasimodo took over Sanctuary II and use it to capture and torment the Silver Surfer and Spider-Man before being ousted by Thanos, who declared the two heroes owed him a debt. Thanos then become trapped in a pocket-dimension that he was investigating. Separated from his source of cosmic energy, he was only able to contact normal reality mentally through a genetically engineered creature called Consumption.
Consumption then contacted the Plunderer, brother of the hero Ka-Zar to serve Thanos, and converted its own body into a gateway to the pocket dimension containing Thanos, destroying itself in the process. The Plunderer entered and contacted Thanos directly, who promised him to ruler-ship over a lifeless Earth. The Plunderer used the terraforming devices of the Antarctic paradise known as the Savage Land to begin to re-shape the Earth, a process, which began to re-power Thanos.
The Plunderer was ultimately confronted by Ka-Zar, but not before Thanos managed to absorb the energies of the pocket-dimension and the terraforming devices. Filled with power, Thanos began to affect the entire galaxy with his newfound primeval energies and opened various portals throughout the universe. Thanos and the Plunderer were forced to pursue Ka-Zar through rapidly shifting areas of the cosmos, as Ka-Zar had stolen the key for the terraforming machines' power. Ultimately, Thanos, tired of the Plunderer and abandoned him on an unidentified world. Thanos' strength, however, had been depleted and Ka-Zar managed to turn the tables on him by knocking him into an active volcano in the Savage Land. Destroying the key to the terraforming machines, Thanos leaped into the device, hoping to recover his source of power, as the device was destroyed by agents of S.H.I.E.L.D.. Thanos was apparently trapped once more in the pocket-dimension.
Soon, however, Thanos discovered a possible exit from the pocket dimension along the mystical Ley-line nexus points. He mentally contacted the monstrous Hulk, who was anguished over the recent death of his wife Betty. Thanos attempted to trick the Hulk into becoming a vessel for Thanos' mind, but was overheard at another Ley-line nexus by Nate Grey, also known as the X-Man. Nate attempted to convince the Hulk to refuse, and ended up locating the Hulk and battling him. Thanos instead used Nate as his physical vessel as the Hulk attacked anew. In the course of battle, the mystical barriers that separated the two dimensions were shattered, freeing Thanos. Ultimately, the X-Man and the Hulk managed to exile Thanos to the pocket dimension yet again.
Later, somehow free of the pocket-dimension, Thanos appeared in Asgard as it lay in ruins and many its warriors defeated by the powerful Mangog. Thanos found an ally in Odin's aide, Tarakis, who helped him to obtain the apocalyptic Map of All-Ending from Odin. Already possessing the Chalice of Ruins, Thanos believed the two objects would help him to find the being known as the Designate. Thanos and Tarakis collected their other ally, Mangog, who had just defeated Thor, and traveled to the planet Rigel-3, home of a peace-loving race of thinkers and dreamers. Once there, Thanos teleported Mangog to the surface, and the hulking behemoth began to destroy everything in sight. Thanos demanded the Fire Gem, the next item required find the Designate in order to stop Mangog's rampage. He was ultimately successful, although Rigel was nearly laid to waste in the process.
Thor promptly traced Thanos route, and allied himself with the Rigellians Recorder and Firelord, the former Herald of Galactus. Thanos and Mangog managed to collect the final items and captured the Designate, a woman named Tarene. As Thor and his allies battled Thanos and his allies, Tarene began to weep for her champion, Thor, an action that granted Thanos the power of Ragnarok. Before Thanos could truly begin Ragnarok, Thor killed Mangog and was given apparel to supply him with the Odin-Force. Thor and his allies battled Thanos once again. As Thanos fell, Tarene wept again for the lives of all those slain by Thanos. As written, she flung her tears upon the fallen Titan, who burst into an all-consuming hellfire, seeming vanquished forever.
In reality, Thanos was not destroyed. He soon confronted Marlo Jones, the wife of Rick Jones, associate of the then-current Captain Marvel. Thanos revealed that Marlo had become the physical refuge for the embodiment of Death and as such, was the target of the death god known as the Walker. Convincing Thor to help them, Thanos, alongside Thor and Captain Marvel, was able to keep the Walker from killing Marlo. Ultimately, Death itself defeated the Walker.