- Stan in "Til Death Do Us Part"
- Poison Ivy's father in "Harley Quinn Highway"
- A student in "Riddle U"
- A guard and Luigi in "All the Best Inmates Have Daddy Issues"
- Leslie in "Dye Hard"
He is known for his role as 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. That same year, he won the Academy Award for Best Adapted Screenplay and received a Golden Globe nomination for the film The Descendants.
Rash also played Mr. Grayson/Stitches in the 2005 film Sky High, Fenton on That '70s Show, Andrew the "whore house guy" on Reno 911! and played Head T.A. Philip in Slackers. He also co-wrote the screenplay for The Descendants (2011), and then co-wrote and co-directed the film The Way Way Back (2013).
Since 2017, he has been the official voice actor for Donald Duck universe character Gyro Gearloose in the reboot of Ducktales.