Precious little is known about Maybourne's background, as his personnel file remains classified. He did begin working with the NID while still a uniformed Air Force officer. Throughout his career, Colonel Maybourne was a controversial figure within the NID, as he frequently pushed for programs which his superiors deemed either unsafe or downright illegal. He also recruited several shady characters for the NID, from officials like Colonel Frank Simmons to secret operatives like Colonel Sean Grieves and Major Dean Newman.
After the promotion of Colonel Martin Kennedy, he took over as the main presence for the NID at Stargate Command. When the Tollan were rescued from their homeworld, he came to take custody of the refugees. Fortunately, SG-1 was able to get the Tollan through the Stargate with the help of Lya of the Nox, much to Maybourne's chagrin. (SG1: "Enigma")
When Teal'c was bitten by an Infestation fly on Svoriin, he came to Stargate Command to take custody of him. He planned to allow Teal'c to change so he could use the bug as a weapon. While en route, Teal'c escaped and Maybourne organized search teams but Colonel Jack O'Neill was able to locate him with the help of Allyson Martin. (SG1: "Bane")
A short time later, he and the rogue elements of the NID moved the Beta Gate to and used it to steal technology including the Touchstone. When SG-1 confronted him at Area 51, he denied any involvement but Colonel Jack O'Neil discovered the Gate they were shown was made of plastic. He confirmed that he was ordered to move the Gate and had SG-1 removed from the facility. Later, SG-1 was able to retrieve the Gate but several NID personnel were stranded offworld. (SG1: "Touchstone")
During a Foothold situation at Stargate Command by the Stragoth, Major Carter contacted him and informed him of the situation. He contacted Stargate Command and they sent the aliens impersonating Dr. Daniel Jackson, Major Paul Davis and Colonel Jack O'Neill to bring her in but only if Maybourne is allowed to search the base. They explained to Carter the spill of Tetrachloroethylene caused her to hallucinate and she agreed to return to the SGC. On the airplane the Mimetic imaging device malfunctioned and they shot the aliens impersonating O'Neill and Davis and took Jackson prisoner. Carter realized the noise from the engine caused the malfunction and headed back to the SGC alone while Maybourne put together a strike team. The strike team entered the base and the Stragoth retreated through the Stargate and blew themselves up when the gate shut down. Maybourne visited the SGC after the incident and graciously gave Carter credit for solving the situation, earning himself a warmer reception from the team as a result. (SG1: "Foothold")
Colonel Maybourne's career in the NID came to a close in 2000 when a sting operation led by Colonel Jack O'Neill revealed that Maybourne had been illegally running off-world operations, and had been stealing advanced technology from some of Earth's alien allies (such as the Asgard, Nox, and Tollan). Maybourne fled before he could be arrested, and remained hidden for some time. (SG1: "Shades of Grey")
He was involved in an attempt by the Russian government to conduct a Stargate program of their own. When a team of scientists opened the water sample from the Water planet, they became possessed by the lifeforms. They attempted to flee through the Stargate but were cut off. When the extreme measures protocol was enacted and nerve gas was released, the lifeforms hid in a freezer to survive. When SG-1 was sent in to investigate, they found him frozen solid. After he thawed the lifeforms, they attempted to flee to the Gate but were stopped by Teal'c. When they realized Maybourne could not get them home, they left his body and entered Teal'c. After he regained control of his body, he explained to Colonel Jack O'Neill what had happened and he was arrested once the situation was resolved. (SG1: "Watergate")
Within months of his capture, Maybourne was briefly released in the custody of Colonel Jack O'Neill, as part of an effort to uncover some of Maybourne's old contacts from the rogue NID that were threatening Major General George S. Hammond into retiring. This lead them to Senator Robert Kinsey where they found evidence of Kinsey's involvement in NID activities. However, once Maybourne finished helping O'Neill, he e-mailed the evidence to himself and threatened Kinsey into getting him transferred to a less secure prison. He managed to escape, and remained on the run for a few years. (SG1: "Chain Reaction")
He returned to the United States when Major Samantha Carter was kidnapped by Adrian Conrad. After initially pointing the finger at Colonel Frank Simmons, he eventually came clean to Colonel Jack O'Neill and told him he sold a Goa'uld symbiote to Conrad for $3 million. He helped O'Neill track down the doctor who took possession of the symbiote, which lead them to Carter's location. He helped O'Neill free Carter and tied up the doctors guarding her. When O'Neill went after Conrad, he escaped. (SG1: "Desperate Measures")
He later came out of hiding to help General Hammond investigate the disappearance of SG-1; traveling to Seattle, he hacked files that revealed that the NID were attempting to artificially create Jaffa from ordinary humans. As he and Hammond were escaping from their NID pursuers, Harry shot Colonel Frank Simmonds in the arm (in vengeance for Simmons having shot O'Neill and framed Maybourne). After secretly traveling to Chulak (with the help of Sergeant Siler), he and Hammond conviced Bra'tac to join them in rescuing SG-1 from the clutches of Nirrti, who had entered an alliance with Simmons to make new Jaffa for herself. Afterwards, at O'Neill's request, he was transported back to Earth by Bra'tac's Jaffa and left at an undisclosed location. However, Hammond's 'alliance' with Maybourne forced him to allow Simmons to go free on this occasion, as Simmons vowed that he would reveal Hammond's contact with Maybourne if Hammond tried to expose his role in the conspiracy. (SG1: "Survival of the Fittest")
When Teal'c was trapped in the Stargate, Maybourne helped O'Neill track down the NID facility in Minot, North Dakota. Once there they raided the facility and discovered the Goa'uld in Adrian Conrad and a tape of it telling Colonel Frank Simmons how to save Teal'c. (SG1: "48 Hours")
Maybourne unexpectedly turned himself in to O'Neill's custody in 2003, claiming that he knew the location of an off-world weapons cache. Insisting that he be brought along, Maybourne partook in what was his first off-world trip. Upon reaching P5X-777, however, Maybourne incapacitated SG-1, and activated the Transport portal. O'Neill grabbed him at the last moment, and was transported along with him to a remote, uninhabited location. With no way of returning to the Stargate, O'Neill and Maybourne remained trapped for several weeks. He began to eat some of the local plant life and began to hallucinate. He was shot twice by O'Neill before the rest of SG-1 realized that they had been transported to a moon in orbit around the planet, and sent a Tok'ra Tel'tak to rescue them. Rather than force Maybourne to return to a life prison sentence, O'Neill arranged for the Tok'ra to find Harry a quiet planet to retire on. (SG1: "Paradise Lost")
Two years after Maybourne and O'Neill parted ways, Stargate Command learned that the planet he was living on had become a target for the Goa'uld Ares. O'Neill (now a Brigadier General in command of the SGC) could not allow Maybourne to fall into the hands of a Goa'uld, as he was privy to a great deal of classified information about Earth. SG-1 traveled to the planet, and discovered that Maybourne had set himself up as King of the local community by translating Ancient ruins with references to the past. However, after nearly two years as community leader, Maybourne had changed. He was unwilling to abandon the people who had entrusted him with their safety, and instead insisted that SG-1 could repel Ares just as they had done to other Goa'uld. SG-1 did indeed succeed in defending the planet, largely due to the discovery of a small Ancient vessel. Maybourne remained on the planet as a ruler. (SG1: "It's Good to Be King")
Despite being devious and good at manipulating others for his own ends, Maybourne has at times shown that he is an ally, having proven his own loyalty to Stargate Command and particularly SG-1 many times.
This is shown when he regularly helps Colonel Jack O'Neill in various situations. He also expresses a personal dislike for Colonel Frank Simmons although Maybourne was the one responsible for recruiting Simmons into the NID in the first place.
On Arkhan's World, it's shown that Maybourne has changed his ways, having become an effective ruler who cares about his people. This is further supported by the fact that Maybourne refuses to leave them even after the threat of Ares had been eliminated. (SG1: "It's Good to Be King")