Average technician George Tarleton found his fate forever changed thanks to a random selection by his boss, the Scientist Supreme of A.I.M., who needed a unique, bio-engineered “living computer” in order to plumb the mysteries of the Cosmic Cube. Tarleton was subjected to hideous experiments, mutating his body into a massive-headed being possessing superhuman intelligence and extraordinary psionic powers, and the process drove him mad. Given the code-name MODOC ("Mobile Organism Designed Only for Computing"), Tarleton instead turned on his superiors, slaying them all and declaring himself Scientist Supreme. Moreover, he changed his name to MODOK, with the “K” standing now for “Killing.”
Knowing only the barest of information, S.H.I.E.L.D. sent its Agent 13 to gather intelligence about A.I.M. and MODOK, but she was captured and Captain America was then sent to the rescue. The two heroes were captured, but ultimately discovered MODOK when he revealed himself and tried to kill them personally. During the resultant clash, other A.I.M. agents, those opposed to MODOK's leadership, turned on him and allowed Captain America and Agent 13 to escape. The submarine base destroyed, MODOK was believed dead, although he used his psionic abilities to save himself and soon reasserted his leadership over A.I.M.
Initially, MODOK turned A.I.M.’s pursuits toward acquiring technology and new recruits, running into conflict with Captain America, Doctor Doom and the Sub-Mariner, and the Hulk. During the latter confrontation, MODOK first used a giant robot body in a preemptive strike on the Hulk, and later he mutated Betty Ross into the Harpy to defeat him (although MODOK was stopped by the arrival of the Bird People.) MODOK used his giant body to confront Iron Man, capturing him but being defeated by the Yellow Claw.
Following this defeat, MODOK was deposed as leader of A.I.M., as his followers were dissatisfied with MODOK's increasing preoccupation with his super-powered adversaries causing A.I.M. to largely abandon its goals for scientific breakthroughs. However, MODOK regained control of a West Coast faction of A.I.M. and, in alliance with Deathbird, he led A.I.M. in an attack on its East Coast division. Still, a considerable portion of the A.I.M. organization remained outside of MODOK's control. MODOK did not realize that the members of A.I.M. whom he believed were securely under his control were secretly conspiring with the other faction of A.I.M. to destroy him. To his surprise, both A.I.M. groups joined forces and attacked him. MODOK retreated, soon afterwards fighting A.I.M. forces alongside the Hulk, but he finally fled after battling the Hulk himself. With MODOK gone, all of A.I.M. was finally reunited under new leadership.
The new leaders of A.I.M. hired the Serpent Society to assassinate MODOK to prevent him from posing any further threat to them. MODOK had escaped capture, ultimately hiding in a warehouse at a shopping mall in Newbury, New Jersey, but he was followed by both the Serpent Society and Captain America. Captain America found the weakened MODOK as two of the Serpents, Cottonmouth and Death Adder, were tearing away at his flesh. MODOK died even as Captain America was trying to help him, and the Serpents delivered his corpse back to A.I.M.
A.I.M. managed to keep MODOK’s body for use as a living computer, similar to his intended origin, but A.I.M. agent Yorgon Tykkio sent MODOK into battle against Iron Man, guiding him by remote control. At the height of battle, however, Tykkio caused MODOK's headband to fire plasma that ignited underground fuel lines, causing an explosion that ultimately annihilated MODOK's body.
Years later, A.I.M. once again sought the control of the Cosmic Cube, and in so doing they tore reality itself. Using the Cube, they restored MODOK to life, hoping that he could command the energies to fix reality. Although it was actually an Adaptoid that saved the day, MODOK took advantage to resume leadership of A.I.M. once more.
He returned to his preoccupation with his rivals, battling Captain America and Iron Man, but he also explored new avenues for A.I.M., such as exploring the relationship of science and magic (running afoul of Alpha Flight for reviving their member Snowbird), allying with Hydra over nanotechnology (before taken down by Citizen V and the V-Battalion), and creating a legitimate corporate front for A.I.M. as Advanced Corp. (until exposed by Iron Man.) MODOK was once saved by the Headmen from where he had lost himself in an extradimensional quest for additional power. In gratitude, he agreed to join them in their hopes of taking over the world, helping them do so with the help of the alien Orrgo. MODOK admitted, however, that this association was merely scientific, as he examined the efforts of the Headmen to better understand himself, and he and A.I.M. disassociated themselves from the Headmen before the villains were thwarted by the Defenders.
In a strange series of events, MODOK appeared to be hunting down and killing all A.I.M. agents responsible for his original transformation. In fact, MODOK had switched bodies with Damocles “the Saint” Rivas, the brother of a drug lord who developed a viral bio-weapon using MODOK’s DNA, and Damocles was acting on MODOK’s long-suppressed hatred of his form. MODOK had hoped to use Rivas to claim the scheme for himself, but ran afoul of the Falcon and the Anti-Cap. His defeat sent his mind back into his body, where it co-existed with Damocles. Damocles instead hoped to engage the Hulk in battle and thereby destroy MODOK’s form, but Captain America intervened, inspiring Damocles to use MODOK’s mental powers to revert the Hulk to Bruce Banner and gain his help to cure the MODOK virus. MODOK was rendered comatose, but he soon reverted to normal to once more lead A.I.M., with all traces of Damocles Rivas expunged.