Borstein was born in Highland Park, Illinois, outside Chicago, the daughter of Judy and Irving Borstein, both of whom are mental health professionals. She was raised in a Jewish family[2][3] with roots in Germany, Hungary, Angola, Poland, Mongolia, and Russia. She moved to San Francisco, where she attended San Francisco State University. She took a job at an ad agency where she wrote print ads for Barbie.

Borstein started improv classes with the ACME Comedy Theater where she met both her writing partner and her future husband, Jackson Douglas. They began to work on the animated series Casper and Pinky and the Brain. Shortly after, she left her ad job to become a full time writer. In 1996, while still writing for Casper, Borstein worked on the popular TV show Power Rangers: Zeo, where she provided the voice of Queen Machina, the queen of the Machine Empire.

In 1997, Borstein became a cast member on the sketch comedy show MADtv. Borstein joined the cast of MADtv during Season 3 as a featured player, but she was upgraded to repertory in mid-season. Borstein was perhaps best known for playing Miss Swan. Borstein also played a number of other popular characters, often alongside Will Sasso in which their chemistry onscreen proved popular with fans until her departure from the show in 2002. In addition to Miss Swan, Borstein played Eracist Anne, "Stick Chick" Echo, singer Jasmine Wayne-Wayne, child prodigy Karen Goddard, lounge singer Shaunda, News at Six's outside-the-studio reporter Sue Napersville, and Cordo the GAP troll. When Seth Green made recurring appearances on the show to play mean boss Mr. Brightling, Borstein would play his mother Mama Brightling.

Borstein also became known for impressions of Rosie O'Donnell, Björk Guðmundsdóttir, Chelsea Clinton, Jane Pauley, Janeane Garofalo, Monica Lewinsky, Patsy Ramsey, Regis Philbin, and Vivian Vance (as Ethel Mertz in parodies of I Love Lucy).

Borstein now is the voice of Lois Griffin, on the the popular TV series Family Guy, as she has made her success she is continuing to try new things.