Mojo is a male, smooth-coat Chihuahua owned by Sam Witwicky. He is a small dog with a very weak bladder, a trait that causes trouble in more than one situation. Mojo is heavily coddled by Sam's mother, Judy, who treats him almost as her own pet rather than Sam's. She adorns him with girl dog jewelry, which she calls his bling, and has placed a sign reading Mojo's Penthouse on his dog house. Sam often complains that this treatment hurts Mojo's self-esteem, which he claims is already heightened due to the embarrassment of being a Chihuahua. In the first film, Mojo breaks his left foreleg, leading to a scene where Sam carries the dog's pain pills and is mistaken for a drug user by a sheriff. Mojo also famously urinates on the Autobot Ironhide's foot, prompting Ironhide to request permission to terminate the rodent; Sam frantically explains that humans love Chihuahuas, and Ironhide spares the dog but worries the urine will cause rust. Later, Mojo is taken into custody by Sector Seven along with the rest of the Witwicky family, but is eventually released.

In Transformers: Revenge of the Fallen, Mojo's leg has healed and he has acquired even more bling. He also gains a new canine companion, a French bulldog named Frankie, whom Mojo easily dominates. As Sam prepares to leave for college, Mojo relieves his stress by repeatedly mounting Frankie on the couch, earning a scolding from Sam's father, Ron. During a surprise attack by animated kitchen appliances, Mojo and Frankie flee from the penthouse when it is fired upon, and later Judy frantically carries both dogs to safety after Sam's car sets the house on fire. Mojo's motivations are purely animalistic: he seeks comfort, attention, and the opportunity to assert dominance over Frankie. His role in the story is largely comic relief, providing small moments of levity and continuity for the Witwicky household. His key relationships are with Sam, who is often exasperated by his mother's treatment of the dog; with Judy, who lavishes affection on him; and with Frankie, his subordinate canine friend. Mojo shows no significant character development between the films, though his physical condition improves. His most notable abilities are his weak bladder, which poses a rust threat to Transformers, and his ability to be easily caught when he escapes the yard. He remains a minor but recurring presence in the Witwicky family's adventures.