The first sketch he wrote for the show was the inaugural iteration of the running gag "Andy’s Little Sister, Stacy," in which Stack's former Chicago improv colleague Amy Poehler portrayed the 13-year-old younger sibling of O'Brien's sidekick, Andy Richter, whose unrequited crush on O'Brien manifests in a range of emotion that runs "from adorable bashfulness to volcanic, homicidal rage."
Stack later said of Poehler in a 2021 Washington Post interview, "I've always been in awe of her incredible talent, but seeing her, Conan and Andy [Richter] have so much fun in that sketch is an especially wonderful memory for me and I'll never forget it.
[citation needed] Among the characters he portrayed: Stack also created the recurring segment "Pierre Bernard's Recliner of Rage",[citation needed] and his voice work on the show included provided the voices of numerous celebrities parodied in the Syncro-Vox faux interviews conducted by O'Brien, including Dick Cheney, Mike Tyson, and Martha Stewart.
Stack played Howard Jorgensen on the NBC sitcom 30 Rock in the episodes "Jack Meets Dennis", "Succession" and "Larry King".
That same year, he appeared as Ted in the TV series Parks and Recreation, a role that recurred into 2014 in the penultimate episode, "Two Funerals", airing on February 17, 2015.
On May 15, 2014, Stack appeared in a sketch titled "Tear Down" in "Craig Robinson Wears a Bordeaux Button Down & Dark Jeans", the second episode of the third season of Comedy Bang!
Stack is a frequent performer in the ASSSSCAT improvisational comedy show at the Upright Citizens Brigade Theater in New York City and Los Angeles.