Sullivan is best known for her six seasons on the sketch comedy series MADtv. She also played the role of Holly Shumpert for five seasons on the CBS sitcom The King of Queens.
She's also voicing several cartoon character roles. She is voicing Mira Nova from Buzz Lightyear of Star Command, Joan of Arc in Clone High. Shego in Kim Possible & Marlene from Penguins of Madagacar TV series.