Since Cody was still too young to run O'Neil Tech, Darius had been appointed chairman of the company until Cody would finally be old enough to take over management.
However, the general public did not know that Darius was contrary to his public image as an exemplary industrialist and caring uncle, involved in criminal machinations and sought sole dominion over O'Neil Tech. To this end, he had Cody under the pretext that the outside world is too dangerous for him, lead a completely isolated life in the O'Neil penthouse, (unknowingly) supported by the robot butler Serling. His goal was to run the company as a gray eminence with Cody as a model figure under his personal control.
But all this began to change, when the Turtles and their mentor Splinter were slyly through a mistake by Cody brought to the future. Cody, who through the stories of his great-grandparents April O'Neil and Casey had heard much of them, took his new guests to live with him and began under their influence to gradually open up to more of the outside world which Darius did not fit into his plan. In addition, the Turtles began to feel an immediate personal aversion to Darius, which was based heavily on reciprocity.
During his time as C.E.O. of O'Neil Tech, Darius utilized his nephew's company for a variety of illicit purposes. Although the full extent of his operations is thus far unknown, Darius has, throughout the series, shown to have a network of cameras to spy on Cody (including one on Serling); employed private soldiers, first with the Inuwashi Gunjin and later on with cloned versions of the Turtles; run a weapon-production and smuggling operation (prohibited under O'Neil Tech's charter) kept under the guise of a recycling program; supplied criminal groups such as Triceraton gangs with said weapons; and perhaps most importantly, stolen plans for Cody's time window. He has also established a resource-sharing alliance with the alien warlord Sh'Okanabo, who supplied him with the evil clones of the TMNT in exchange for the Time Window plans.
The Turtles, who never trusted Darius because of his shady nature, were hard pressed to expose his criminal activities, but could initially prove nothing since his position and cunning allowed him to remain clean. Eventually the Turtles and Cody discovered the gun factory at the bottom of O'Neil Tech.
After being exposed by Cody and the Turtles that he was in fact making weapons, he dropped his innocent façade and decided to openly face them when they had found out the truth. Even with more firepower, Cody managed to defeat his uncle using his Turtle X armor while the turtles defeated their dark counterparts once more.
He is seen hiding out with the Dark Turtles after this, as they rescued him after being defeated by Cody. He sees them merely as his minions though, easily controlling them using his battle armor as well as having placed hypnotic suggestions in them so they can't hurt him physically.
In an attempt to finally finish off Cody, Darius took control of Turtle-X with a Trojan Horse Virus he installed while searching through Cody's database. He captured Cody inside his own machine and tortured the young boy by having all of his artifacts destroyed. However, Splinter and Raphael tracked his location and with his armor damaged, Darius had the Turtle-X go on a rampage until it overloaded but the process was stopped by Serling who uploaded the anti-virus program into himself and fought to install it into Turtle-X, becoming severely damaged due to this.
Darius once again orders the Dark Turtles to destroy the real Turtles. When they fail yet again and tosses them a steak to eat, growling that he hopes they choke on it.
It is assumed he was eventually taken down off screen
- "Bishop to Knight"
- "Fly Me to the Moon"
- "Invasion of the Body Snatcher!"
- "Bad Blood"
- "Timing is Everything"
- "The Fall of Darius Dunn"
- "Turtle X-Tinction"
- "DNA is Thicker than Water"