He appeared on the HBO program The Wire as defense attorney Maurice Levy.
Kostroff is best known for his five seasons playing drug lawyer Maury Levy on the HBO series The Wire, and from his recurring appearances on shows including The Blacklist, Billions, The Good Wife, and Law & Order: Special Victims Unit, as well as his role as Marshall Pike on the Disney Channel series Sonny with a Chance.
He played Peter Madoff, brother of Bernie Madoff (Robert De Niro), in HBO's The Wizard of Lies, and Shepsie Tirschwell in David Simon's 2020 miniseries The Plot Against America.
Kostroff is the author of four books: Letters from Backstage (a chronicle of his time on the road with two Broadway tours);[3] The Stage Actor's Handbook: Traditions, Protocols, and Etiquette for the Working and Aspiring Professional; Audition Psych 101 (based on his workshop of the same name); and Advice from 'The Working Actor' (a textbook based on the “Working Actor” advice column in the weekly publication Backstage; Kostroff was also one of the writers of that column).
In September 2024, he was cast in a recurring role on the upcoming superhero series Spider-Noir.