"Penny Peterson is a double threat-cute and smart. Penny rules her elementary school untile she encounters Sherman, who is even more of a "brain" than she is. But Penny's natural charisma and daring, as well as her caring and loyalty, draw Sherman into a friendship that truly stands the test of time."
Penny first appears in the background on Sherman's first day of school she is seeing Sherman and Peabody bid each other goodbye. She and some other girl are also seen making fun of Sherman.
In class, Penny answers a question on George Washington correctly, but Sherman answers it better and with bonus answers. This causes Penny to start to despise Sherman a lot because she wants to be smarter and better than everyone else.
At lunch, Penny humiliates Sherman in front of the whole school; she throws his sandwich, treats him like a animal, calls him a dog and takes his special dog whistle. Sherman gets so furious at her that he bites her arm. This leads to a physical fight between the two and Peabody to have an unfriendly confrontation with Ms. Grunion, a mean social worker who threatens to reclaim Sherman and blames Peabody for Sherman’s behavior.
The next night, Peabody invites her and her parents over for supper to make amends for Sherman's behavior at school so Penny's parents won't get Sherman taken away. In Sherman's room, Penny and Sherman refuse to speak to each other and threatens to beat up Sherman if he talks to her. Penny threatens Sherman to tell her how he knows all that stuff about George Washington which makes Sherman accidentally tell Penny about the WABAC. While traveling, he takes Penny to ancient Egypt and she refuses to go home.
When Peabody and Sherman try to get Penny back, she refuses to go home with them because she is going to be married to King Tut, who she plots to rob when he dies young. When she finds out that she'll be killed when King Tut dies and turned into a mummy, she decides to go home but the Pharaoh refuses to let her go. Now, Peabody and Sherman must get her back.
After Penny is saved, the WABAC runs out of fuel and Peabody has to go to Leonardo Da Vinci to get more fuel. She and Sherman run out to play and fly Da Vinci's plane. They have a great time, but Mr. Peabody does not approve.
When they are about to go home, Penny blurts out that Grunion is going to take Sherman from Peabody. This causes Peabody and Sherman have an argument, which ends with Peabody accidentally insulting Sherman.
They end up ancient Greece and Sherman runs away to join the Trojan War to prove himself to Peabody. After Peabody saves Sherman, Penny is in danger of falling off a cliff and Sherman saves her but it also causes Peabody to fall off a cliff. When Penny and Sherman think Peabody is dead, they start crying, not knowing that Mr. Peabody is actually holding onto a branch from the cliff.
Sherman decides to go back to when he lost Penny so they can prevent Peabody from going back in time. But, this causes trouble: the future Peabody is shown to be alive, there are now two Shermans, and Grunion has arrived to take Sherman. Just as Grunion is about to take Sherman away, Penny admits it's her fault and apologizes to Sherman, but Grunion declines. But, the two Shermans touch each other, morph into one, and cause the laws of space and time to be all jumbled up.
When Grunion is about to take Peabody to animal control for biting her, Sherman and Penny stop her and have Washington, Clinton, and Lincoln to give Peabody a presidential pardon. Mr. Peabody is freed and is allowed to keep Sherman. When the timeline is being messed up even more, Sherman suggests that he and Peabody go to the future and fix everything. Penny puts all her faith in Sherman to save the universe. The universe is saved and Grunion is taken to ancient Greece forever.
Despite having a major role in the movie, Penny and her family are largely absent in the show. However, there were a few mentions of her from time to time.
In the episode, Brain Switch / Koikawa Harumachi, Penny is briefly mentioned by Mr. Peabody who asks Sherman how she's doing during an idle conversation, which Sherman simply replies “good.”
In the episode, Seen It / Edgar Allan Poe, a man holds a sign on the window that says "Where's Penny?" This was probably to poke fun at fans who asked this question.
Penny is very rich and spoiled. She appeared to have been envious of Sherman's smarts and called him a dog and bullied him which resulted into a fight. She may also have a wicked sense of humor and spiteful as she would go far to get back at those she dislikes, as she bullied Sherman and in Egypt, when King Tut threatens to kill Mr. Peabody and Sherman, she says "You'd do that for me?", indicating she may have wanted that to happen out of spite for Sherman showing her up in class. However, later they form a very strong friendship during their adventure.
She also is very rebellious, as she tends to stray from Sherman and Mr. Peabody during the time traveling adventures, as shown when she wanted to stay in Ancient Egypt to rule with King Tut and keep his riches once he died, and when she took Da Vinci's flying machine for a test run with Sherman as an unwilling accomplice.
She is also manipulative and greedy, as shown when she wanted to marry Tut knowing he would die young in order to inherit his throne and riches (though eventually backed out once she realized when he died she would be killed and die with him) and when she tricked Sherman into showing her how Da Vinci's machine worked.
Despite all of this, she is also shown to be willing to face up to her mistakes, as she proclaimed in front of everyone that it was her fault for provoking Sherman to bite her and that neither he or Peabody did anything wrong and attempts to take full responsibility for her actions in order for Mr. Peabody and Sherman to remain together (though this is not enough to convince the cold hearted Ms. Grunion to release Mr. Peabody) and apologizes for everything she has done. This proves not only can she understand her own mistakes, but is also loyal and caring to those she is close to.
Penny is a cute and tomboyish 8-year old little girl with long blonde hair, blue eyes, and freckles. She wears a black headband. She also wears a light pink wrist-length shirt under a pink sleeveless dress with dark pink stripes and a black belt around it, white tube socks, and black Mary Jane shoes. She seems to wear lip gloss and black eyeliner.
- During an interview at the movie's premiere night, Ariel Winter said that Penny had "liked" Sherman since she first met him, but didn't realize it at the time.
- Penny's minor mentions in The Mr. Peabody & Sherman Show implies that even though she has made no appearances whatsoever, she still exists within the show's universe.
- It is also implied that the show runners were aware of Penny's absence and are possibly teasing the fans who ask for her, as evidenced with a man holding a "Where's Penny?" sign during the episode, "Seen It / Edgar Allan Poe."