[Information taken directly from Wikipedia]
The Leader made his first appearance in Tales to Astonish #62 (Dec. 1964)
Born Samuel Sterns in Boise, Idaho, he worked for a chemical plant there in a menial capacity. While moving radioactive materials into an underground storage area, some of the radioactive materials exploded, bombarding Sterns with gamma radiation. He recovered, and found that the radiation had changed him from an ordinary human into a green-skinned, super-intelligent entity with an oversized brain housed in a towering cranium. As was the case with most individuals mutated by gamma radiation early-on, the particular set of characteristics Sterns acquired by exposure to it were said to result from a subconscious desire; in his case, the desire to be as smart as his brother Philip, who was a physicist in the employ of the same facility. Calling himself the Leader, Sterns embarked on various ambitious criminal schemes, with the Hulk as his primary nemesis, consistently backed by a self-constructed army of super-strong, virtually invulnerable plastic Humanoids. These included an attempt to rewrite the Earth's history by infecting the primordial ooze with gamma radiation in order to recreate society in his image, with himself as its ruler.
The Leader began his career as a would-be conqueror by creating a spy ring to overthrow the United States federal government. The Leader sends a spy to steal a robot Banner was developing. Banner goes after the spy but turns into the Hulk. The spy overpowers an unsuspecting watchman, leaving him bound and gagged while he gets into the robot and drives it to the test. A guard finds the watchman who reports he was knocked out, however they can't bring down the robot, and neither can the Hulk. The spy finds Banner's caves and makes a missile to fire at the base. Finally in the caves Hulk knocks the robot into a deep pit, killing the spy. He then stopped the missile. The Leader then dispatched his ally the Chameleon to capture the Hulk for study by impersonating Bruce Banner. Although the Chameleon failed, the Leader succeeded in capturing the Hulk by sending a horde of Humanoids to subdue him. He also succeeded in stealing the Absorbatron, a device invented by Banner that could absorb the energy of a nuclear explosion, and thereby render any nation that possessed it immune to nuclear attack. The Hulk eventually escaped the Leader and destroyed most of his lab, along with the Absorbatron, but the Leader himself escaped. Eager to sell his Humanoids to hostile nations, the Leader gave a demonstration of his power by activating a 500-foot (150 m)-tall Humanoid and ordering it to attack a nearby missile base. The Hulk eventually encountered the Humanoid and managed to thwart the Leader's plans. Curious to learn more about the power of the Hulk, the Leader then rescued the Hulk from the Army, who had cornered the monster in a cave. After operating on the Hulk to save his life, the Leader reminded the Hulk that he was in his debt. Using this leverage, the Leader sent the Hulk to the homeworld of the Watcher to raid the Watcher's "Ultimate Machine", a device containing all the knowledge in the universe. However, this huge amount of knowledge proved too much for the Leader. He collapsed to the ground, unmoving, and the Hulk assumed that he died of shock. General Ross soon discovered the Leader's abandoned base and accidentally activated the "Hulk Killer" a large Humanoid designed to defeat the Hulk.
Later, the Leader reappeared and offered General Ross aid in neutralizing the Hulk. The Leader dispatched the Super-Humanoid to assist the General in seizing control of America's nuclear arsenal. The Leader then hijacked the U.S. Army's Murder Module vehicle, and used it against the Hulk. He freed the Rhino from captivity and sent him against the Hulk. After briefly turning back into Samuel Sterns to question Banner about the Hulk's possible weaknesses, he resurrected the Glob to try to pit him against the Hulk. The Leader later sent mental projections of the Rhino, Xeron, Namor, the Missing Link, and others against the Hulk, but then suffered a mental breakdown. The Leader then created android duplicates of the President, Vice President, and military personnel in an attempt to kidnap the real President and Vice President. He then superimposed his consciousness upon the Rhino in order to battle the Hulk. He later used the Hulk and the Thing as pawns in a contest with Kurrgo.
Some time later, the Leader briefly took over Gamma Base. Some time after that, he gamma-irradiated Manhattan's water supply in an attempt to mutate humanity like himself. He later activated Arsenal, and then dispatched the Avengers through time.
After a period of time, the gamma radiation in his body began to wear off. At first, the Leader attributed his lapses in concentration to overworking his mind finding ways to defeat his greatest enemy (the Hulk). By the time the Leader realized what was happening, much of the intelligence that could have solved his plight was gone and texts that were once child's play to him were now hopelessly beyond him (he even forgot the access code of his secret base). In this period, he made cash however he could by means fair or foul, until he managed to convince the Gray Hulk to help him regain his intelligence by promising that he would help the latter to remain the Hulk full-time (instead of only at night).