During her visit to her aunt Mrs. Weaver's house, a next-door neighbor of the Flynn-Fletcher family, she overheard Candace describing Phineas and Ferb's antics, and it quickly escalated into a point of pride that her brothers were more obnoxious than Candace's. She acted as lookout and cheerleader for her brothers during their fort-building competition with Phineas and Ferb. In the end, after Phineas and Ferb beat her brothers in the fort building contest, she admits that Candace's brothers are more obnoxious than her own brothers ("Thaddeus and Thor").
She owns a "Stop The Busting" program ("Perry The Actorpus"), which helps people to stop trying to bust their siblings. Phineas and Ferb gave her a machine to help Candace stop having a busting obsession. She let the former slip which led to Candace destroying the machine with a lead pipe.
Mandy is as tall as Candace. She wears a light blue sleeveless t-shirt and navy blue jeans. She has curly brown hair.
- Candace's nickname "Candy" may have been the source of Mandy's name.
- She somewhat resembles Candace due to the long neck.
- She is voiced by Aliki Theofilopoulos Grafft, a storyboard artist and writer on Phineas and Ferb from 2006 to 2014.
- There was a scene with Mandy in the storyboards for Excaliferb, but it got cut.
- "Thaddeus and Thor" (First appearance)
- "Perry The Actorpus"
- "Great Balls Of Water" (Non-speaking cameo)
- "Phineas and Ferb The Movie: Candace Against the Universe (Cameo)