- Actor: Chester
Although Mojo is Sam's dog, Judy appears to consider him her pet as well. She put a "Mojo's Penthouse" sign on his dog house and puts girl dog jewelry on him, claiming that it's his "bling". Sam argued that it was hurting his self-esteem issues, which he claimed were already heightened due to the fact that he was a Chihuahua.
Mojo broke his left foreleg shortly before Sam got his new car. Sam was carrying Mojo's pain pills when he was taken down to police headquarters, which made the overzealous officer there think Sam was a junkie.
When the Autobots went to the Witwicky residence, Mojo made the mistake of releasing lubricant on Ironhide's foot. Upon discovering this, Ironhide requested permission to terminate the "rodent," but Sam Witwicky frantically told Ironhide that humans love Chihuahuas. Ironhide agreed to spare it, complaining that the lubricant would cause "rust".
Mojo was taken into custody with the rest of the family by Sector Seven, with Judy threatening to kick the ass of anyone who hurt him, but Mojo was later released with the Witwickys. The agent who had to escort Mojo to the van with a catch pole used to control vicious or stray dogs must have felt really silly.
- At one of the BotCon 2007 movie writers panels, it was revealed that Mojo was originally going to be a Bulldog (as one of the writers owned a Bulldog named Mojo growing up), but director Michael Bay decided to change the dog to a Chihuahua.
- Mojo's broken leg was caused by Judy accidentally loading him into the washing-machine along with a pile of clothes. This is only mentioned in the novelization, however, as dialog about it was cut from the final version of the film.