In his twenties, he performed and taught comedy at the Upright Citizens Brigade (UCB). It was working at the UCB that led Mulligan to working at CollegeHumor, (premiering on December 14, 2017, with Who's the Real Cop?). He has appeared on, and is an executive producer of, the comedy panel game show Game Changer and the spin-off improv show Make Some Noise on Dropout. He has also appeared on other Dropout shows such as Very Important People and Gastronauts.
In March 2026, Mulligan was a guest in "Dungeons & Dealers" for the television series Ted (2024); Mulligan helped rewrite the episode on set. That month, Mulligan also appeared on the new web series Jeopardy! YouTube Edition as a contestant.
Since 2018, Mulligan has been an executive producer, writer, and gamemaster for Dimension 20 on Dropout (which also hosts his video podcast, Adventuring Academy). In his capacity as a professional D&D gamemaster, he has been cited by Bleeding Cool, Comic Book Resources, Wired, and Wizards of the Coast's own Dungeons & Dragons podcast—Dragon Talk. Glen Weldon, in a review of Dimension 20 for NPR, commented that "Mulligan is such a good DM and he's got so many improv skills. He's such a close and responsive listener that no matter what the players throw at him, he can always roll with it, without breaking the game. And that is a very rare skill, so it's terrific stuff".
Mulligan was the Dungeon Master for Exandria Unlimited: Calamity, a weekly, four-part actual play web series spin-off of Critical Role that premiered on May 26, 2022. He returned to Critical Role in 2024 as the Dungeon Master of the three-part special titled "Downfall" for the show's third campaign. Mulligan was then the Dungeon Master for Exandria Unlimited: Divergence which premiered on February 13, 2025.
In January 2023, it was announced that Mulligan, along with Erika Ishii, Aabria Iyengar, and Lou Wilson, would star in the creator-owned actual play podcast Worlds Beyond Number. The show launched on March 1, 2023, and quickly became one of the top 20 Patreon accounts in the world; Mulligan was the "inaugural game master". The first book of this campaign ran for four seasons with the finale airing in August 2025. The next campaign premiered in March 2026 with Iyengar as the game master and Mulligan shifting to a player role.
In August 2025, Critical Role announced that Mulligan would take over as Game Master from Matthew Mercer for their fourth campaign. The new campaign premiered on October 2, 2025 and features a new world "outside its usual Exandria setting". Rolling Stone stated that "the inclusion of Mulligan is huge, but not entirely surprising" given Mulligan's background as "a well-established Game Master" who created the "titanic success" Dimension 20 along with previous comments made by Mercer on eventually shifting "into a 'Professor X' role as a mentor to the next generation of storytellers rather than remaining the face of the brand in perpetuity". Also in October 2025, Mulligan was the Dungeon Master for the Endless Dungeon tour in Australia.
