In the first film he is 9 years old, and he is shy. Mark feels left out of his family, as he looks different and claims that he doesn't fit in. His nickname of the film was Fed-Ex, given by his older sister Sarah. He gets bullied at his new school. Mark has a frog named Beans who dies, so he runs away towards the end of the movie. When Mark is found, Jessica explains that his appearance and differences could have been given to any of his siblings. Jake says that "without [him], they wouldn't be the twelve Bakers anymore; they'd be eleven." Mark returns home and has a closer relationship with his family. Mark is not shy after returning home.
In the second film he is 11 years old and more confident and no longer shy. Mark volunteers at the animal shelter. He likes animals. He doesn't appear to have a pet frog though. Mark joins the family at Lake Winetka and be friends Kenneth Murtaugh, who gets in trouble for trashing a tennis court. Mark competes in the wheelbarrow race during the Labor Day Cup and wins the event for his family. When Nora goes into labor, Mark leads them to a hospital with Kenneth by following a scat trail (animal poop).