Much of Sullivan's early stage experience came in Chicago, where he received improv training at the iO Theater.

[6] After appearing in the original Broadway cast of Lombardi, Sullivan joined the long-running revival of Chicago in 2011.

He has starred in single episodes of various TV series: A Gifted Man (2012), Elementary, Law & Order: Special Victims Unit, and The Americans (2013).

For this role, Sullivan has received two nominations for the Primetime Emmy Award for Outstanding Supporting Actor in a Drama Series.

He provided the voice of the camel in the popular "Hump Day" ad for American auto insurance company GEICO.