He gained national exposure for his portrayal of characters on Late Night with Conan O'Brien and as Kenneth Parcell in 30 Rock.

McBrayer has recurring roles in Phineas and Ferb, Puppy Dog Pals, Amphibia, The Middle and in the Netflix series Big Mouth.

For his episode role in Your Pretty Face is Going to Hell, he received a Primetime Emmy Award for Outstanding Actor in a Short Form Comedy or Drama Series nomination.

[5] From 1995 to 2002, he worked at The Second City and IO Theater in Chicago,[4] an experience that introduced him to 30 Rock creator Tina Fey and prepared him for his role on her show.

[1] McBrayer appeared in over 80 sketches on Late Night with Conan O'Brien (2002–04), often lampooning his own Southern upbringing by playing stereotypical hillbilly characters.

In 2012, McBrayer lent his voice to the Disney animated film Wreck-It Ralph, playing the title character's video game opponent and eventual friend Fix-It Felix Jr. More recently, he has starred alongside Los Angeles Clippers player Blake Griffin in Kia commercials, and in 2013 appeared in a commercial for Barnes & Noble.

In September 2016, on the season 10 premiere episode of The Big Bang Theory, McBrayer played Penny's older brother Randall.

[12] In December 2018, it was announced McBrayer would make his West End stage debut in the transfer of the Broadway musical Waitress, playing the role of Ogie, until June 15, 2019.