Critical Role
Mercer is the Dungeon Master of the web series Critical Role, which launched on Geek & Sundry in 2015, where he leads several other voice actors through a Dungeons & Dragons campaign. Critical Role was both the Webby Winner and the People's Voice Winner in the "Games (Video Series & Channels)" category at the 2019 Webby Awards; the show was also both a Finalist and the Audience Honor Winner in the "Games" category at the 2019 Shorty Awards. After becoming hugely successful, the Critical Role cast left the Geek & Sundry network in early 2019 and set up their own production company, Critical Role Productions; Mercer is the company's chief creative officer. Soon after, they aimed to raise $750,000 on Kickstarter to create an animated series of their first campaign, but ended up raising over $11 million. This feat went on to set a Guinness World Record for the most-funded TV series project in Kickstarter's history. In November 2019, Amazon Prime Video announced that they had acquired the streaming rights to this animated series, now titled The Legend of Vox Machina; Mercer reprised his role as Sylas Briarwood and other characters.
Luke Winkie, in a human-interest story for Slate, called Mercer "an expert improviser" and "a maestro with the dice in his hands, weaving interlocking plot lines and complex thematic threads out of thin air" with "the innate ability to make a Dungeons & Dragons campaign feel like a tightly wound limited drama". Winkie commented that "it can be downright intimidating to watch Mercer when he's at the peak of his powers. As a career voice actor, he possesses all of the subtle performance intangibles that saturate his storytelling with life: the timing, the verve, the language flourishes". Academics Zac Boyd and Míša Hejná, in the journal Language in Society, highlighted that Mercer "introduced 1,144 unique NPC characters during" the second campaign of Critical Role. They analyzed nineteen characters based on exceeding a threshold of minimum voice time and plot relevance and determined through "holistic analysis of voice quality" of these characters that "breathiness" in Mercer's voice "emerged as signalling positive morality and stances of safety, comfort, and trust, where whisperiness signals negative morality and stances of threat. Qualitatively, pitch dynamism was also found to correlate with morality and stancetaking: the more limited the pitch dynamism, the more likely it is that the character portrayed is an Enemy and that they adopt stances of threat".
In October 2020, Mercer became the creative advisor for the Critical Role board and card game imprint called Darrington Press. From June to August 2021, Mercer appeared on Exandria Unlimited, a spinoff of Critical Role, as a player. In March 2022, he reprised his role in the two part special Exandria Unlimited: Kymal. In 2025, he was the game master for the Age of Umbra limited series. In August 2025, Critical Role announced that Brennan Lee Mulligan would take over as Game Master from Mercer for their fourth campaign, which premiered on October 2, 2025; Mercer appears in the campaign as a player. Rolling Stone stated that "the inclusion of Mulligan is huge, but not entirely surprising" given Mulligan's background as "a well-established Game Master" 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".
