After college, Walsh took improvisational comedy classes in Chicago, where he became a regular performer at the Annoyance Theater and ImprovOlympic, and studied under Del Close. In 1991, he met comedian Matt Besser, with whom he began performing stand-up comedy. Along with Matt Besser, Amy Poehler, and Ian Roberts, he is a founding member of the Upright Citizens Brigade improv comedy troupe and played "Trotter" in the troupe's sketch comedy series, which ran for three seasons on Comedy Central. Before the show, the UCB comedy foursome had already been doing improv shows at their theatre in New York, which used to be a burlesque house before they remodeled it to become a theatre. To date, they have founded four successful Upright Citizens Brigade comedy theaters located in New York at UCB Chelsea and The East Village UCB, and Los Angeles with UCB Franklin and UCB Sunset.
Besides his sketch work with the Upright Citizens Brigade, Walsh is also known for his memorable supporting roles in popular comedy films in recent years including Old School, Starsky & Hutch, Be Kind Rewind, Ted, and Keeping Up with the Joneses. Walsh also wrote and starred in the cult indie-comedy Martin & Orloff. Walsh was a correspondent on The Daily Show from 2001 to 2002, and made regular appearances in comedy sketches on Late Night with Conan O'Brien and MTV's Human Giant as well as starring in the Comedy Central mockumentary series Dog Bites Man in the summer of 2006. Walsh has the distinction of having appeared in several Todd Phillips comedy films, most often playing characters aptly named "Walsh".
In 2010, Walsh created the improvised comedy series Players for Spike TV. Walsh (creator, writer, director and executive producer of the show) stars in the series alongside his Upright Citizens Brigade partner Ian Roberts, as two brothers who run a sports bar together. The show aired for one season, concluding on August 14, 2010.[3]
Walsh hosted the podcast Bear Down[4] with comedian Scot Armstrong (screenwriter of Old School) where they discuss their favorite football team The Chicago Bears and do sports-themed comedy bits such as fake call-ins and in-character interviews. The podcast has also featured appearances by comedians such as Horatio Sanz, Ian Roberts, Joe Nunez, and Matt Price.
Walsh directed his first feature film High Road, an indie-comedy he co-wrote with Josh Weiner.[5] The film was released on March 6, 2012.[6] Walsh co-wrote his second film, A Better You, with Brian Huskey, who also stars in the film.[7]