Stein was born in Marquette, Michigan, and grew up in Milwaukee, Wisconsin.
[2][4] Following graduation from Juilliard, Mary got her first screen acting job in the "Time's Arrow" episode of Star Trek: The Next Generation, playing an alien nurse.
As her career developed, she played roles such as Miss Floom, the landlady and owner of the Flealands Hotel in Babe: Pig in the City and Miss Rue Who in How the Grinch Stole Christmas.
Some of her well-known television roles include Ms. Sneed on General Hospital and Gerda in Providence.
She also appeared in Clint Eastwood's film Changeling, alongside Angelina Jolie.