He may be best known for his roles as Charles Brooks in Younger, Trevor Langan in Law & Order: Special Victims Unit, and Jack Boyle in Blue Bloods.

He has made appearances in several commercials, such as the anti-tobacco "Fair Enough" ads, and has been the face for Persil ProClean detergent since the late 2010s.

[9] Hermann was in the Broadway premiere of Eric Bogosian's Talk Radio, starring Liev Schreiber, and featuring fellow Law & Order: Special Victims Unit cast member Stephanie March.

[11][12] He also guested on 30 Rock during its first season and starred in ABC's mid-season replacement series Cashmere Mafia as Davis Draper.

While appearing on Law & Order: Special Victims Unit, Hermann met lead actress Mariska Hargitay, whom he married on August 28, 2004, in Santa Barbara, California.