Mientus, who also appeared on "Smash," will play a hearing impaired and openly gay DC Comics villain Pied Piper, also known as Hartley Rathaway.
The actor is set to appear in episodes 11-12 of "The Flash," which premiered Oct. 7.
"The Flash" is developed by writer/producers Greg Berlanti, Andrew Kreisberg and Geoff Johns. It is based on the DC Comics character Flash (Barry Allen).
Mientus has also been seen Off-Broadway in Carrie.