[1] Ellis grew up in New York and studied painting at the University of Montana,[2] where she was influenced by art history, particularly Egon Schiele, Max Beckmann, and German Expressionism.
[3] Her first illustration work began when she met Colin Meloy at the University of Montana and designed gig posters for his college band Tarkio.
[9] She is also well known for her work with the indie folk rock band The Decemberists,[10] for whom she has created album art, T-shirts, websites, posters, and stage sets.
In 2015, Ellis wrote and illustrated her first solo children's book, Home, which explores different types of domiciles around the world, underwater, and in space.
[13][14][3] Ellis' second solo children's book, Du Iz Tak?, published in 2016, is written entirely in an invented "bug language".
[17] Ellis created the cover and interior illustrations for Meloy's 2017 middle grade book The Whiz Mob and the Grenadine Kid, published by HarperCollins.
The Wildwood Chronicles, illustrated by Ellis and written by her husband, musician Colin Meloy, was a bestselling series for middle-grade readers.