He often works with Dropout (formerly CollegeHumor) as a writer, performer, and producer, and he is the creator and regular gamemaster for its Dungeons & Dragons-based actual play series Dimension 20.

[4] In an interview with the Times-Herald Record, a 16-year-old Mulligan expressed conviction in theatrical gaming's potential to enable "kids [to] come out healthier beings than when they first arrive".

[12] Mulligan was also a writer for several scripted shows on Dropout, including the Star Wars parody space saga Troopers,[13] and the mockumentary Gods of Food.

[14][15] In 2023, Mulligan was a writer for Foul Play, an interactive murder mystery series created by Andrew Barth Feldman and Alex Boniello.

[2] In his capacity as a professional D&D gamemaster, he has been cited by Bleeding Cool,[20] Comic Book Resources,[21] Wired,[22] and Wizards of the Coast's own Dungeons & Dragons podcast—Dragon Talk.

[23] 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.

[28][29] In January 2023, it was announced that Mulligan, along with Erika Ishii, Aabria Iyengar, and Lou Wilson, will star in the creator-owned actual play podcast Worlds Beyond Number.

[41] In August 2023, during the 2023 Writers Guild of America and SAG-AFTRA strikes, Mulligan led a Dungeons & Dragons-themed picket day at Universal Studios where the final boss was AMPTiamat.