"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 naturnal 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.
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 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 Tutankhamun who has become her boyfriend (much to Sherman's fury). 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 and possibly develop feelings for each other.
When they are about to go home, Penny blurts out that Grunion is gonna take Sherman from Peabody. This causes Peabody and Sherman have an argument.
They end up ancient Greece and Sherman runs away to join the Trojan War, which proves to be a huge mistake. 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 beleiving they have lost their dog.
Sherman decides to go back to when he lost Penny so they can be with Peabody. 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 says it's her fault and apologizes to Sherman, but Grunion declines. But, the two Shermans touch each other, morph into one, and cause the space time continum 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. 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.
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. Sherman has a large crush on her. Penny also seems to have a crush on Sherman as well. So it happens that she and Sherman have a crush on each other. 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 he didn't do 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 young 8-year old girl with long blonde hair and blue eyes. 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.
- In the third season episode of The Mr. Peabody and Sherman Show", 'Brain Switch/Koikawa Harumachi' Penny is briefly mentioned by Mr. Peabody who asks Sherman how she's doing, which Sherman simply replies 'good'.
- In the fourth season episode of The Mr. Peabody and Sherman Show, 'Seen It/Edgar Allen Poe a man holds a sign on the window that says Where's Penny?
- This implies that even though she's yet to make an appearance, Penny still exists within the show's universe.