Originally an ordinary mouse, Monterey was one of five lab animals kept by Black Heron of F.O.W.L.. In the course of her experiments with the Third Eye Diamond, she granted them all super intelligence, and they promptly fashioned and began wearing clothes and walking upright. They also seem to have recognized the evil of Heron and her associates and/or developed a desire for freedom. This led them to escape her lab and construct a flying machine for transport, during which incident they met and befriended Launchpad.
After helping him escape F.O.W.L. Headquarters and thwart a scheme by Steelbeak, Monty and his companions left for parts unknown. They were eventually recaptured by F.O.W.L. and imprisoned at the Library of Alexandria along with many other beings and relics associated with adventure. They joined in cheering on Launchpad as he once again faced off with Steelbeak. After being liberated, they were among the allies of the Duck family to join them in a final confrontation with Bradford Buzzard.
Behind the scenes
The DuckTales 2017 version of Monterey Jack's design and upcoming appearance were teased in 2019 at a convention.