Rash played "Mr. Grayson/Stitches", sidekick to supervillain Royal Pain, in the 2005 film Sky High. He played Fenton on That '70s Show and Andrew (the "whore house guy") on Reno 911!. He guest starred in the final episode of Friends, and played Head T.A. Philip in Slackers.

Rash and comedy partner Nat Faxon moved into screenwriting, writing a pilot in 2005 for a series entitled Adopted, about an adult who finds out his parents are not his birth parents. The show did not take off. From 2009 until the show's finale in 2015, Rash starred on Community as Craig Pelton, the dean of the community college in which the show takes place.

Rash and Faxon wrote the screenplay for The Descendants, based on the novel of the same name by Kaui Hart Hemmings. The script appeared on the 2008 edition of the Black List, which lists the most popular unproduced scripts in Hollywood at that time. The film was produced in Hawaii and starred George Clooney; it was released on November 18, 2011 to critical acclaim. The film received a Golden Globe nomination and won an Academy Award for Best Adapted Screenplay. Rash and Faxon co-wrote and directed the film The Way Way Back, which received a standing ovation at its premiere at the 2013 Sundance Film Festival. Parts of the film are based on Rash's teenage life. Rash is also a member of the Los Angeles-based improvisational and sketch comedy troupe The Groundlings. Since 2017, he has been the official voice actor for Donald Duck universe character Gyro Gearloose in the reboot of Ducktales, taking over the role from Hal Smith who died in 1994.

Article Courtesy of Wikipedia.