Casnoff began in the theatre, although many of his strongest roles were not until the late 80s, when he had already broken into film and TV. In 1988, he won a Theatre World Award for his work as the American champion in the Broadway production "Chess" and the next year was nominated for a Drama Desk Award for his work in the New York Shakespeare Festival production of "Up Against It", with music by Todd Rundgren. He later starred on Broadway as Blackthorne in the short-lived "Shogun: The Musical."

Casnoff broke into TV in the early 80s appearing in the "American Playhouse" production of "King of America" (PBS, 1982). In 1984, Casnoff had a small role in an ABC Afterschool Special "Out of Step" and played Lafayette in the "George Washington" miniseries for CBS. He had a far more challenging role in the miniseries "North and South" (ABC, 1985), playing Elkanah Bent, the enemy of Patrick Swayze, a role Casnoff reprised in "North and South Book II" (ABC, 1986). Casnoff was the dashing was hero Lt. Guilford in "Ironclads" (TNT, 1991), and then played "Sinatra".

Casnoff's feature film work has been sporadic. He made his debut in the forgettable Japanese-funded "Message From Space" (1978) and co-starred in the unsuccessful campus comedy "G.O.R.P." (1980). He also appeared in the direct-to-video release "Temptation" (1994).