Ellis was born in Laurel, Mississippi. She graduated from Southern Methodist University in 2002.
Ellis married her It's Always Sunny in Philadelphia co-star Charlie Day on March 4, 2006. Their first child, a son named Russell Wallace Day, was born on December 15, 2011.
In 2008, she co-wrote and starred in the independent film A Quiet Little Marriage. Ellis and Artemis Pebdani wrote and starred together 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, California. In addition, Ellis and Pebdani are members of the Los Angeles-based performance group Discount Cruise to Hell.
Ellis was a series regular on the NBC romantic comedy series Perfect Couples, which ran for one season in 2011. Ellis also stars in The Lonely Island's music video for the song "Diaper Money".
In 2015 Ellis got her first main role as Debbie Sanderson, across Fred Savage and Rob Lowe in The Grinder (TV series).