Thanos made his first comic appearance in Iron Man #55 (Feb. 1973)
Thanos was born on Saturn's moon Titan, and is the child of Eternals Mentor and Sui-San. Thanos was carrying the Deviants gene, and as such shared the physical appearance of the Eternals' cousin race. At the time of his birth, his mother, Sui-San, attempted to kill the Titan at first sight. The doctors then sedate his mother at Eternal Mentor's request. During his school years, Thanos has the reputation of the smartest child in school as well as a pacifist, though he kept himself isolated due to his appearance and only played with his brother Eros, though soon enough, a group of children approach him and he becomes friends with them despite his appearance. He was also a talented artist and had recurring nightmares about someone trying to kill him at birth, which is revealed to be his mother who appears to be imprisoned in a mental institution. Thanos is then tasked to dissect a lizard, which he runs away from and vomits outside, where he is encountered by a girl who tells him about a cave she visits. Thanos visits the caves with his new friends, where they are trapped by collapsing rocks. After three days of digging out the rocks, Thanos sees that a group of lizards have eaten his friends right down to the bone and he flees. Upon returning to school, he is once again alone due to the circumstances surrounding him being the only survivor. Thanos' present day dialogue says the people of Titan had decided that Thanos had murdered them, starting him on the road to his present day self. The truth is, however, Thanos was convinced by the same girl who showed him the cave, to drop his overly passive nature so he could kill the lizards in revenge for his dead friends.
For the most part of his early life, Thanos was treated fairly, however he became mindful of his appearance and isolated himself, only playing with his brother Eros (Starfox) and pets. By adolescence, Thanos had become fascinated with nihilism and death, and worshipped and eventually fell in love with the physical embodiment of death, Mistress Death, albeit under a different name and appearance. Committing several horrifying acts at her command, he was doomed to a future of destruction and hate. As an adult, Thanos attempted to create a new life for himself, by starting a family and supporting them by joining a crew of vicious space pirates. One day however, Thanos was visited again by Mistress Death and gave into the temptation she posed. He murdered his family and took command of the pirate crew and set off to his old homeworld of Titan. After many augmentations such as the use of bionics and mysticism, Thanos became the most powerful of the Titans soon becoming referred to as the Mad Titan.
Wishing to impress Mistress Death, Thanos gathers an army of villainous aliens and begins a nuclear bombardment of Titan that kills millions of his race. Seeking universal power in the form of the Cosmic Cube, Thanos travels to Earth. Prior to landing, his vessel destroys a nearby car as a family witnesses his arrival. Unbeknownst to Thanos, two of the family members in the vehicle survive: the father's spirit is preserved by the Titanian cosmic entity Kronos and is given a new form as Drax the Destroyer while the daughter is found by Thanos' father Mentor and is raised to become the heroine Moondragon. Thanos eventually locates the Cube, and also attracts the attention of Mistress Death. Willing the Cube to make him omnipotent, Thanos then discards the Cube. He imprisons Kronos and taunts Kree hero Captain Marvel, who with the aid of superhero team the Avengers and ISAAC (a super-computer based on Titan), is eventually able to defeat Thanos by destroying the Cube.
Thanos later comes to the aid of Adam Warlock in a war against the Magus and his religious empire. During this alliance Thanos cultivates a plan to reunite with Mistress Death, and secretly siphons off the energies of Warlock's Soul Gem, and combines these with the power of the other Infinity Gems to create a weapon capable of destroying a star. Warlock summons the Avengers and Captain Marvel to stop Thanos, although the plan is foiled when Thanos kills Warlock. The Titan regroups and captures the heroes, who are freed by Spider-Man and the Thing. Thanos is finally stopped by Warlock, whose spirit emerges from the Soul Gem and turns the Titan to stone. Thanos's spirit eventually reappears to accompany a dying Captain Marvel's soul into the realm of Death.