Psychiatrist Dr. Leonard Samson came to General "Thunderbolt" Ross with a plan to cure Ross's daughter Betty, who had been transformed into a crystalline form through a blood transfusion with the Sandman. Samson had technicians devise a cathexis ray projector that would drain off gamma particle energy and psionic energy from Bruce Banner as he transformed into the monstrous Hulk whom General Ross had long hunted. Samson theorized that as a result of this process, Banner would be cured of the physiological instability that continually transformed him into the Hulk. Hoping to save Betty and to be cured himself. Banner willingly participated in the experiment. The cathexis ray succeeded in turning Betty Ross back to normal, employing Banner's psionic energy.
However, most of the psionic energy drained from Banner as well as a considerable amount of his excess gamma radiation remained within the cathexis ray projector. Motivated by scientific curiosity, and probably by a desire to gain superhuman powers for himself, Samson irradiated himself with the energies remaining within the projector. The result was that the slight, brown-haired psychiatrist was transformed into a massively muscled, green-haired superhuman with gamma-boosted strength. Banner himself seemed permanently cured, but he emotionally broke under the strain of seeing Betty, the woman he loved, being romanced by Samson, and exposed himself to gamma radiation, thereby undoing the cure.
Samson later lost his superhuman physique and power when he took a blast of concentrated gamma radiation intended for the Hulk. However, months later Samson regained his gamma-mutated form and superhuman strength when he was bombarded by intensive radiation from an exploding gammatron, an experimental gamma-ray generator. He has remained in this form ever since. Doc Samson, as he has been nicknamed, has long sought to capture and permanently cure the Hulk. Samson worked as a university teacher during the recent time that the Hulk was traveling through different dimension, unable to return to Earth. But Samson returned to his pursuit of the Hulk once the monster reappeared on Earth.
The Hulk hasn't been Samson's only focus although Banner is his primary center of attention. Samson was integral in the capture of the first known gamma-powered serial killer, Patchwork, as well aiding others who have been affected by gamma radiation such as the Initiative recruit, Geiger. Doc Samson had also been trying to help Penance of the Thunderbolts cope with his participation in the Civil War|tragedy in Stamford as well as delay, if not destroy, the rise of Norman Osborn.
At an undisclosed time, Samson was kidnapped by A.I.M. and was used as an experiment by M.O.D.O.K. which resulted in him developing a split personality. For a time, Samson switched back and forth between his good and evil self, but his corrupted side eventually won the struggle for dominance and claimed the part of him that was Leonard is dead. He has joined a group of villainous masterminds known as the Intelligencia, and his greatest desire seemed to be the need to kill his former ally, the Red Hulk, thereby proving his superiority. He maintained contact with the superhero community as they were not aware, as of yet, of his shift in personality.
Samson remained an integral part of the Intelligencia's plans as he was the key to manipulating the Red She-Hulk to do their bidding. His involvement in her brainwashing also proved to help save her life after she was stabbed through the chest by the son of Hulk, Skaar. Just seeing Samson's face was enough to enrage Betty to trigger the transformation and permit her healing factor to repair her injuries. Still, even with his participation that led to the Intelligencia's master stroke, something changed within him as he witnessed Banner sacrifice his humanity in order to save everyone. Samson attempted to help Banner siphon the immense amounts of gamma radiation back from the Hulk army as well as the "Hulked-Out Heroes", but his body couldn't take the strain. Samson willingly gave his life, but Leonard died a hero.