[2] He is best known for his role as Murray Bauman in Netflix's horror-supernatural series Stranger Things and as Martin in the BBC comedy Fleabag.

As a voice actor, he has contributed to television series Jeff & Some Aliens and TripTank on Comedy Central and, more recently, I Heart Arlo and Inside Job on Netflix, among others.

[4] While in New York, Gelman was a regular performer at the Upright Citizens Brigade Theatre, as part of the duo Cracked Out and sketch group Mr.

He has co-starred as Chris Elliott's sidekick Brett Mobley in the live-action Adult Swim series Eagleheart and is a regular sketch performer on HBO's Funny or Die Presents and Comedy Central's Nick Swardson's Pretend Time.

He has had recurring roles on The Life & Times of Tim and The Inbetweeners, as well as making guest appearances on comedy programs such as Bored to Death, Curb Your Enthusiasm, The Office, Happy Endings, Aqua Teen Hunger Force, and The League.

[13][14][15][16] In 2017, Gelman starred in and co-wrote Lemon, opposite Judy Greer, Michael Cera, Nia Long and Gillian Jacobs, directed by Janicza Bravo.

[19] In 2016, it was announced that Gelman was cast in the second season of the hit Netflix science-fiction web series Stranger Things as Murray Bauman, a conspiracy theorist and former journalist.

[22] Gelman and frequent writing partner and collaborator Jon Daly performed for many years as the comedy rap duo "Cracked Out".

[23] Gelman has gained recognition for appearing as the "Little Bit Of Luck" character in the "Take 5" New York Lottery ad campaign that ran from 2008 through 2011.

In November 2023, Gelman was among more than a dozen creators and celebrities who confronted TikTok executives in a public letter claiming that the app is directly amplifying the spread of antisemitism.