Mitchell, 29, is best known for playing Lucy Camden on the drama series "7th Heaven."
She is to replace former "Blossom" star Mayim Bialik in the role of school counselor on "Secret," TVGuide.com said, adding Mitchell's episodes are to start airing in January.
The Web site did not say why Bialik is leaving the show.
Gal Gadot cast as Wonder Woman for 'Batman vs. Superman'
Kate Middleton recycles dress at movie premiere