Cornelius is a scientist who believed that apes and humans are actually related to each other in a since. He was quite hesitant to believe Taylor's story of being from another world or that he had any intelligence. However he was soon convinced of the latter when Taylor revealed he can write and even read a map. Although Taylor's claims of coming from some vessel from space baffled him, because apes have no concept of flight or space travel. However Taylor surprised both Cornelius and Zira by building them a paper airplane and tossing it through the air.
It was soon discovered that Zira was housing Taylor and by his order, he had Taylor taken back to the animal hospital. Taylor made his escape from Ape City, but was ultimately captured again. However all of Ape City soon learned that he can speak when he uttered, "Get your stinkin' paws off me, you damn dirty apes." Soon a hearing was made for Taylor and Cornelius and Zira tried to prove to the Ape Council of Taylor's intelligence. However he was quickly dismissed and some believed that the chimps simply augmented Taylor's mind themselves.
While Cornelius proved loyalty to Dr. Zaius, he reluctantly helped Zira free Taylor and a human female named Nova. There Cornelius, and Zira had horses for Taylor and Nova. Then they went to explore the Forbidden Zone against Zaius' orders. The group who had been aided by Zira's nephew Lucius made to a cave in the Forbidden Zone, but Zaius and his gorilla soldiers got there too. Cornelius managed to prove his theory about man and ape, but Doctor Zaius forbid Cornelius from revealing it. He believed that if the ape people knew that ape was once second to humans then the entire society would delve into chaos. He reluctantly agreed and so all his findings were destroyed.
Cornelius and Zira after Taylor had disappeared soon met another human from Taylor's time. Cornelius and Zira report that they haven't seen Taylor since their last encounter with him, but help the human John Brent get out of the city so Brent can find him.
Cornelius, Zira, and a colleague Doctor Milo went to the Forbidden Zone to find Taylor's spacecraft. It was repaired and they escaped the Planet of the Apes before it was destroyed by the Alpha-Omega bomb. They landed in the 1970's and soon became celebrities. Unfortunately their colleague Milo was accidently killed by a gorilla of that time period. Both apes soon learned of the humans of this time period and were fascinated, but also relayed their future of humanities fall and that had to change. Cornelius and Zira were expecting a child, but soon the three of them were hunted by the military. Both along with a different ape baby that they took to protect their own child were killed.