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