Blanchard's 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, in which she played Melanie Schlegel. She also starred in the television series 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 on the television series Clueless. She also played Roxanne on the television series 7th Heaven from 2002 to 2004.
Blanchard received critical acclaim for her performance as Nancy, the American girlfriend of main character Jeremy Usborne, in the award-winning second series of English sitcom Peep Show. She reprised this role in the fourth series. She played Sally on the HBO show Flight of the Conchords and has a recurring role on the 2014 FX crime series Fargo.