Penelope is introduced in School Lies as one of Blair Waldorf's minions. In A Thin Line Between Chuck and Nate, Penelope dethrones Blair after it comes out that she slept with both Chuck Bass and Nate Archibald. She also allows Jenny Humphrey to officially become one of the girls of the Met steps. Afterwards, Penelope plays a part in bullying Blair after everyone returns to school from spring break. After it comes out that Jenny stole a $15,000 dress from Hazel Williams' dress, she sides with Blair but eventually chooses to follow Jenny after she brings Nate to a reservation at the restaurant Butter; knowing Penelope has a crush on him (The Blair Bitch Project). When it comes out that Jenny lied about having sex with her boyfriend, Asher Hornsby, because he's gay, Penelope and the other girls abandon her to follow Blair again (All About My Brother).
In The Ex-Files, Penelope and Isabel begin bullying Jenny, which results in her cutting school to work at Waldorf Designs atelier instead. Meanwhile, Dan Humphrey has begun seeing Amanda Lasher, a new girl at Constance. Chuck suggests a way to get Amanda out of the picture, for Serena's benefit, and Penelope takes him up on his suggestion. She uses a hair loss solution to burn hair off Amanda's head but it's revealed that Chuck set Amanda up with Dan to make Serena jealous and want to rule Constance again. As a result, Blair is dethroned as queen once again and Penelope and the girls begin following Serena.
When it's time for the Snowflake Ball, Penelope decides to hire Jenny to make her a dress. While being fitted, both girls receive a Gossip Girl blast that Nate will be attending the ball with Vanessa Abrams. Angry at Vanessa for seeing Nate behind her back, Jenny agrees to help Penelope, Hazel, and Isabel embarrass her at the ball. Jenny gives Vanessa a dress to wear with zero lining, while Penelope, Hazel, and Isabel set up the lights to expose her underwear in front of everyone. However, Jenny feels guilty and tries to save Vanessa but fails (It's a Wonderful Lie). In In the Realm of the Basses, Jenny returns to Constance after being home schooled and discovers how poorly Penelope treats Nelly Yuki. After Jenny confronts her at a frozen yogurt shop, Penelope calls her father and reports Jenny for bullying her. They decide to hold a trial, with Blair as judge, but Blair is uninterested; as she is preoccupied with Chuck's return to New York. As a result, Penelope and the girls feel abandoned by Blair as their queen. Later, Jenny is able to extort information on Penelope, Hazel, and Isabel from Nelly and uses it to blackmail them into being nicer to Nelly. However, the girls misunderstand and think she was trying to become queen and they offer the position to her. But Jenny is uninterested and realizes Nelly was using her to outrank the other girls under a new queen. When Jenny declines the offer, Nelly goes back to following the other girls around.
In You've Got Yale!, Penelope helps Blair haze new Shakespeare teacher, Rachel Carr. After Blair is caught and sentenced to detention, she asks Penelope, Hazel, Isabel, and Nelly to help her get revenge. All four girls refuse until a new policy, spearheaded by Rachel, is unveiled: disallowing all cell phone usage during school. They then comply and help Blair get revenge (Carrnal Knowledge). In Remains of the J, Penelope plans to throw a party but cancels once Serena throws a party for Jenny that lots of socialites will be at. At prom, Penelope, Hazel, and Isabel scheme to embarrass Blair by fixing the Prom Queen vote so that Nelly wins. However, Chuck catches onto their plan and stops it before it takes place. During the season finale, The Goodbye Gossip Girl, Penelope and the others refuse to help Serena in her plan to take down Gossip Girl. She isn't seen again until season four.
Penelope makes her next appearance in The Undergraduates, on the first day of school at Columbia University. She's revealed to be a member of the ultra exclusive sorority, Hamilton House. However, she falls back in to being one of Blair's minions. In Shattered Bass, she attends Blair and Louis Grimaldi's engagement party.
In I Am Number Nine, Blair holds a competition for potential bridesmaids, with Penelope, Kati Farkas, Jessica Leitenberg, and Charlie Rhodes as contestants. Her final challenge is seeing who can be the first to kiss Nate. Charlie successfully tricks Penelope into believing Nate thought she was one of the hottest girls at Constance and wished he had hooked up with her. She tries to kiss him but he ducks away, causing Penelope to embrace a waiter. In Father and the Bride, Penelope attends Blair's bachelorette party and helps Beatrice Grimaldi get Blair drunk. At Blair's wedding in G.G., she serves as a bridesmaid.
When Blair's diary is released, Blair asks Penelope to find Serena's deepest secret to expose. When she fails, Blair fires her. After being fired, Serena asks her to keep Blair away from the Shepherds' divorce party so Dan believes she doesn't care about him. However, Penelope fails at this. This is her last appearance in the series.
- She attended summer school at The Sorbonne.
- She's had a crush on Nate since the 20th century.
- She's a National Merit Scholar.
- When she was a freshman, she attended prom with a junior but when another girl started flirting with him, she burned her hair off with a "Nair-tini".
- During her senior year of high school she was sleeping with her dad's junior business partner.
- Her great aunt was a founding member of Hamilton House.
- In Blair's diary, Penelope is shamed for being pathetic due to her always attending events alone.