A native of Highland Park, Illinois, Borstein is a graduate of San Francisco State University, where she studied rhetoric. She was trained in improvisational comedy at the ACME Comedy Theatre, near Hollywood, California, and was selected to join MADtv after being scouted by talent agents who noticed her work at the theatre. She was a writer and voice actor for several television shows, including Casper, Pinky and the Brain and Power Rangers: Zeo, before joining the cast of MADtv as a featured player, and later as a repertory player in 1997.

As an actress, she has made appearances in such films as The Lizzie McGuire Movie, Catwoman and Dinner for Schmucks.