From 1997-2001, Cline performed his original work at the Austin Poetry Slam venues. He was the Austin Poetry Slam Champ in 1998 and 2001, and competed on the Austin Poetry Slam Teams at the 1998 Austin National Poetry Slam and the 2001 Seattle National Poetry Slam. His most popular spoken word pieces include: "Dance, Monkeys, Dance", "Nerd Porn Auteur" and "When I Was a Kid." Paulo Ang, a UCSD student, created a popular flash cartoon out of Ernie's track "Dance Monkeys Dance." Cline himself subsequently reworked "Dance Monkeys Dance" into a faux educational filmstrip, which became a popular viral video that has now been translated into 29 different languages.

As of 2008, Cline has published a book of poetry, The Importance of Being Ernest and released a CD of spoken word, The Geek Wants Out, both through The Wordsmith Press.

In 1996, Cline wrote a sequel (Buckaroo Banzai Against the World Crime League) to W. D. Richter's 1984 film The Adventures of Buckaroo Banzai Across the 8th Dimension and made it available on the Internet.

In 1998, Cline's screenplay, Fanboys, generated local interest in Austin, TX, including mention on Harry Knowles' website Ain't It Cool News. In late 2005, the Weinstein Company purchased Ernest Cline's script for Fanboys , casting Sam Huntington, Chris Marquette, Dan Fogler, Jay Baruchel and Kristen Bell as the five main characters. At the 2008 San Diego ComiCon, Fanboys producer Kevin Spacey announced the release date of September 19, 2008. However, The Weinstein Company changed the release date to November 26, 2008. Fanboys was released on February 6, 2009.

Also in the summer of 2008, Lakeshore Entertainment announced that they would be producing Cline's screenplay, Thundercade. Thundercade follows the story of a video game junkie in his mid-30's who learns that a young gamer has beaten a record he set when he was a teenager, who then travels with his friends to the world's largest gaming championship, Thundercade, to restore his former glory.

In June 2010 Cline sold his first novel Ready Player One in a bidding war to the Crown Publishing Group (a division of Random House). The film rights to the novel were sold the following day to Warner Bros. with Cline attached to write the screenplay.