Garth was a Starfleet officer who rose to the rank of fleet captain, and whose exploits in exploring uncharted worlds became required reading at the Starfleet academy. He was a personal hero of Captain Kirk. However, an accident that left him badly maimed caused him to go insane. After being nursed back to health by the peaceful beings of Antos IV, and gifted a technique of cellular metamorphosis to heal his mangled body, he offered them the galaxy as repayment. When they refused, his diseased mind saw it as a great insult, and he attempted to commit genocide against their whole species. His people naturally refused, and he was sent to an asylum for the criminally insane on Elba II.
On Elba II, he expanded the technique that the beings on Antos IV had given him to the point that he could make himself appear to be someone else. He used this to trick a guard into letting him out of his cell, and soon took over the asylum, dubbing himself "Lord" Garth. While in control, he manufactured incredible powerful chemical explosives (so much so that a single canister could destroy a whole planet), and turned a rehabilitation chair into a torture device that could be used indefinitely, and tortured the former governor of the asylum, Donald Cory, with it.
When the USS Enterprise came to Elba II for delivery of new medicines, Garth took it as an opportunity to escape. He took the guise of Governor Cory at first, and trapped Kirk and Spock in a cell with the real Cory. He declared his ambitions for galactic conquest, but was unable to beam aboard the Enterprise because he didn't know the code, a chess move, to gain access. Over dinner, Garth attempted to get the code out of Kirk and Spock, and when that didn't work, he demonstrated his explosives on his own consort. Garth finally attempted to fool Spock by taking Kirk's form and attempting to escape with him, but Spock eventually was able to identify his true captain.
Garth was subdued and his followers returned to confinement. After being given some of the new drug, Garth appeared to finally be brought back to his old self, and even shook Kirk's hand.
In the novel Garth of Izar the new treatments progressed well. Garth had progressed well enough that he was released in 2269. He then travelled to Antos IV and infiltrated a group of rebels operating on the planet.
The following year Garth had returned to Earth. Admiral José I. Mendez declared Garth fit for space duty and assigned him to travel on the Enterprise to Antos IV and attempt to repair the sociological damage he had done earlier. Garth succeeded in his mission - convincing the rebels to stop using their shape shifting abilities. Garth was subsequently named the Federation's Ambassador to Antos IV, making a personal, lifelong commitment to that world.
In the Star Trek comics continuity Garth was released after the treatments worked. However Starfleet did not trust him enough to give him another command. They tried to steer him towards teaching, but Garth rejected that as being too menial for someone like him. He stole a ship and fled for deep space.
Garth suffered a relapse in his mental illness. He stole the starship USS Zephyr after murdering Captain Burroughs. He renamed the ship Renegade and using a crew of mercenaries attacked Klingon outposts. The Enterprise was sent to investigate. Despite attempting to kill Kirk several times Garth was recaptured and sent back to Elba II for further treatments.