After a brief stint as a model with Ford Models and Rascal's Agency, Muth pursued a career in acting, studying at the Lee Strasberg Theatre and Film Institute in New York City, and getting her first professional experience doing commercials in 1993. Her first major role was in 'the 1995 film Dolores Claiborne. She followed that with a role in the eighth season premiere of Law & Order.
Ellen Muth, a 19-year-old actress who looks considerably younger, virtually explodes in the role of Constance, the volatile 13-year-old only child of divorced parents. An emotional powder keg who one minute can be as clinging as a baby and the next delights in cruelly demolishing the nearest grownup with laser-like sarcasm, Constance is a flailing mood swing on a circuitous warpath. The film ... crackles dangerously to life whenever Constance (who narrates the film) is on the screen with her father Hank (Terry Kinney).
In 2000, her work included appearances in episodes of The Beat and Law & Order: Special Victims Unit, as well as a role in The American Collection adaptation of "Cora Unashamed", a short story by Langston Hughes from his 1934 collection The Ways of White Folks. Muth also had a guest role in "Don't Ask", the original unaired pilot episode of the 2000 Fox show eventually airing as Normal, Ohio, but the show was revamped, resulting in the elimination of her role.
Muth's most widely known work was as the star of the 2003-4 Showtime television series Dead Like Me, where she played Georgia Lass, one of a team of reapers led by Rube Sofer played by Mandy Patinkin.
Muth's more recent roles have been lower in profile. She appeared in Jack 'n' Jill, a 2007 MFA thesis film. She voiced the character of Addie Vost in the first animated short of Tofu the Vegan Zombie and a character in the audio dramatization "Anne Manx in the Empress Blair Project". In September 2008 she joined three other actors at the Theatre Artist Workshop in Norwalk, Connecticut in a reading of Fleece the Flock, an original musical comedy in development and directed by Joel Vig.
Muth was born in Milford, Connecticut, the daughter of Rachel and Erich Muth.
Muth was one of two actresses who shared a best supporting actress award at the 1995 Tokyo International Film Festival, recognition Muth earned for her screen debut in Dolores Claiborne. In 1999 she won the American Film Institute (AFI) Best Actress Award for The Young Girl and the Monsoon.
In 2004, Muth was nominated twice for Dead Like Me. This included a Saturn Award nomination in the Best Actress in a Television Series category. She was nominated for a Satellite Award in the category of Best Performance by an Actress in a Drama series.