Born and raised just outside of Philadelphia, Brennan became interested in comedy at a very young age. He watched shows like "Late Night with David Letterman" (NBC 1982-1993), as well as the standup specials of Richard Pryor, and emulate their acts verbatim to friends and family. After high school, Brennan moved to New York City to attend film school. He dropped out after a year, however, and immediately set his sights on becoming a standup. He eventually landed a job as a doorman at a comedy club in the Greenwich Village neighborhood of New York City where he met a bright young comic named Dave Chappelle. The two became fast friends and before long Brennan was writing jokes for him. In the mid-'90s, Brennan moved to Los Angeles to pursue his comedy-writing career in earnest. He soon landed a job writing for MTV's "Singled Out," as well the Nickelodeon comedies "Keenan & Kel" (1996-2000) and "All That" (1994-2005). It was during this time that Brennan and Chappelle started writing a stoner comedy together called "Half Baked." Their script was made into a movie in 1998, and was universally panned by critics. Brennan and Chappelle did not speak to one another for an entire year after the film's failure. However, that would all change with "Chappelle's Show." The show debuted in January of 2003 on Comedy Central and was an immediate hit, spawning several notable sketches, including "Charlie Murphy's True Hollywood Stories" and "Wayne Brady's Show." By the end of the second season in 2004, "Chappelle's Show" was Comedy Central's highest-rated program, with the show's star signing a $50 million contract for two more seasons. But for reasons still unclear, Dave Chappelle abruptly walked off the set during filming of the third season and flew to Africa. Given no forewarning about his departure, Brennan felt betrayed and angry. It was the last time Brennan spoke to his former creative partner. Throughout the remainder of the decade, Brennan performed standup at clubs throughout Los Angeles, while lending his comedy writing talents to shows like "Funny or Die Presents" (HBO 2010). In addition to launching a comedy podcast in 2011, Brennan also served as a temporary host for "Attack of the Show!" (G4 2005-2013) in 2012.