Much of Sullivan's early stage experience came in Chicago, where he received improv training at the
iO Theater. Among his Chicago stage experiences were in the cast of
Defending the Caveman at the Lakeshore Theater and the cast of
The Ballad of Emmett Till at the
Goodman Theatre. After appearing in the original Broadway cast of
Lombardi, Sullivan joined the long-running revival of
Chicago in 2011. He first gained attention for his portrayal of Tom Cleary on the
Cinemax drama
The Knick. He also played
Taserface in
Guardians of the Galaxy Vol. 2 and Benny Hammond in
Stranger Things. He has starred in single episodes of various TV series:
A Gifted Man (2012),
Elementary,
Law & Order: Special Victims Unit, and
The Americans (2013). He also played Sprit in the
2008 Sundance Film Festival nominated movie
North Starr, and appeared in such films as
The Normal Heart and
The Drop (2014), the latter with
The Knick co-star
Jeremy Bobb. In March 2022, Sullivan was set to lead comedy pilot
The Son In Law at ABC. In April 2024, Sullivan joined
Amazon MGM Studios' sci-fi thriller entitled
Mercy. ==Personal life==