Casnoff began his career touring in the 1972–73 national company of Godspell where he understudied Jesus and Judas. He was cast as understudy for the title role in the 1976 Broadway production of Rockabye Hamlet, a rock musical version of Shakespeare's play, but the show closed after only seven performances.
In April of 1988, Casnoff starred as Freddie, an arrogant American professional chess player in the musical Chess. The show garnered poor reviews and little audience interest and closed three months later.
Casnoff created the lead role of John Blackmore in Shogun: The Musical (1990), based on the popular book and miniseries by James Clavell.
Casnoff's first major role was that of Elkanah Bent in the popular 1985 miniseries North and South. He went on to reprise the role in two sequels which aired in 1986 and 1994.
In 1992, he was chosen to play Frank Sinatra in the television miniseries Sinatra. Casnoff met Sinatra during shooting and went on to receive a Golden Globe nomination for his performance.
Casnoff portrayed Russian criminal Nikolai Stanislofsky on HBO's acclaimed TV series Oz from 1999 to 2000.
In 2000, he joined the cast of Lifetime Television's Strong Medicine. As Chief of Staff Dr. Robert Jackson, Casnoff stayed with the show for five seasons, until 2005. He also directed an episode and went on direct an episode of Monk.
In 2008, Casnoff's first starring role in a feature film, Sight Unseen, will be completed, with no further details on a release date.
Casnoff graduated from Central High School in June 1967. He has been married to actress Roxanne Hart since 1984. They have two sons, Alexander and Macklin.
