He played Dean Craig Pelton on the NBC sitcom Community (2009–2015), for which he was nominated for the Critics' Choice Television Award for Best Supporting Actor in a Comedy Series in 2012.

Rash and comedy partner Nat Faxon moved into screenwriting with a pilot in 2005 for a series entitled Adopted, which did not take off.

[5][7] They wrote the screenplay for The Descendants (2011), based on the novel of the same name, which appeared on the 2008 edition of the Black List (the most popular unproduced scripts in Hollywood at that time).

[8] The film was released to critical acclaim,[3] receiving a Golden Globe nomination and winning the Academy Award for Best Adapted Screenplay.

[7] The ensemble comedy starred Steve Carell, Toni Collette, Allison Janney, Maya Rudolph, and Sam Rockwell.

In 2020, Rash and Faxon reunited for Downhill, an adaptation of the film Force Majeure (2014) by Swedish director Ruben Östlund.