Brennan is currently a fulltime cast member and writer at CollegeHumor. He has made appearances in CollegeHumor shorts and sketches, most notably as various company CEOs, as well as Dropout originals such as Um, Actually and The Rank Room. At CollegeHumor, Brennan is most notable for being the creator and DM of Dimension 20 and Dimension 20 sidequests. He also hosts the podcast Adventuring Academy which is about Dungeons and Dragons. He also briefly played a character named Deadeye in Brian Murphy and Emily Axford's Not Another DND Podcast.
In addition to CollegeHumor, Brennan is also the writer of the webcomic Strong Female Protagonist. He also a staff member at The Wayfinder Experience, a live-action roleplaying camp for kids and teens. Brennan also performs regularly at UCB theater.
"HE'S NOT CURSED! HE'S JUST A GUY! Some people have to be normal. Do you know that most people if you stab them with a sword will die right away?" ― Brennan Lee Mulligan, Dimension 20 LIVE – Havoc at the Hotel Cavalier
"Your name is "Hilda Hilda" and you live at 22 Hilda Street and 22 Hilda Boulevard?!" ― Brennan Lee Mulligan, Dimension 20 LIVE – Heartache on the Celestine Sea
"Oh good, you speak English. These next two minutes are going to be very crucial for you... what do you want me to tell your family?" ― Brennan Lee Mulligan as Evan Kelmp, Misfits and Magic
"I hate having my shit fucked." ― Brennan Lee Mulligan, ACoC Episode 4