Mary Elizabeth Ellis (born: May 11, 1979) is an American actress and writer. She is known for her role as The Waitress on the FX sitcom It's Always Sunny in Philadelphia. Ellis also starred in the NBC sitcom Perfect Couples and the Fox sitcom The Grinder. She also appeared as Nick's ex-girlfriend in the sitcom New Girl. From 2017 to 2019, she starred as Lisa Palmer on the Netflix horror-comedy Santa Clarita Diet.