Mary Elizabeth Ellis (born May 11, 1979) is an American actress. She is best known for her recurring roles as The Waitress on the FX comedy It's Always Sunny in Philadelphia (2005–present), Nick's ex-girlfriend Caroline in the Fox sitcom New Girl (2011–2018), and Lisa Palmer on the Netflix horror-comedy Santa Clarita Diet (2017–2019). She starred as Amy in the shortlived NBC sitcom Perfect Couples (2010–2011) and Debbie Sanderson in the Fox sitcom The Grinder (2015–2016). More recently, she played Gary's mother Anita in Licorice Pizza (2021), directed by Paul Thomas Anderson.

Mary Elizabeth Ellis was born on May 11, 1979, in Laurel, Mississippi.

She graduated from Southern Methodist University in 2001.

Since the first season of the FX/FXX comedy series It's Always Sunny in Philadelphia began airing in 2005, Ellis has been mostly known for her role as The Waitress.

In 2008, she co-wrote and starred as Olive in the independent film A Quiet Little Marriage. Ellis was a series regular on the NBC romantic comedy series Perfect Couples, which ran for one season in 2011.

In 2015, she got her first main role as Debbie Sanderson, co-starring with Fred Savage and Rob Lowe in The Grinder. In 2020, she appeared in Disney's Godmothered. The next year, she appeared in Licorice Pizza (2021) as Momma Anita.

Ellis has appeared in a number of other television series, including Reno 911!, House M.D., Without a Trace, Cold Case, New Girl, Happy Endings, and Netflix's Santa Clarita Diet. In 2024 she appeared in A Man on the Inside as Ted Danson's daughter. The series was renewed for a second season, which was released in November 2025.

Ellis and her It's Always Sunny colleague Artemis Pebdani were as of 2011 members of the Los Angeles-based performance group Discount Cruise to Hell. Together, they wrote and starred in the stage show Mother May I Dance with Mary Jane's Fist?: A Lifetime Original Play at the Upright Citizens Brigade Theater in Los Angeles. The stage show was later adapted to TV for Adult Swim in 2018.