Her career was launched with a part in a McDonald's commercial, and as an eight-year-old on the Canadian children's show The Kids of Degrassi Street, where she played Melanie Schlegel. She also starred in War of the Worlds as Suzanne McCullough's daughter Debi and in Nickelodeon's Are You Afraid of the Dark? as Kristen. Blanchard is probably best known for playing the part of Cher Horowitz (originally portrayed by Alicia Silverstone) on the television series Clueless based on the 1995 movie of the same name.

She received critical acclaim for her performance in the award-winning second series of British sitcom Peep Show. She reprised this role in the fourth series (2007).