After starring as Tzeitel in the 1964 Broadway production of Fiddler on the Roof, Merlin became a theatrical casting director and a television and film actress. Her best known screen credits include Judge Lena Petrovsky on Law & Order: Special Victims Unit and Mrs. Arujo in Mystic Pizza.
After earning a bachelor's degree from the University of California, Los Angeles, she trained with acting teacher Michael Chekhov. She first appeared on screen in the 1956 film The Ten Commandments; her professional stage debut was in the 1961 Broadway production of Becket.
She taught in the Master of Fine Arts-Acting program at New York University's Tisch School of the Arts.
Born Joann Ratner, she grew up in Chicago, Illinois as the daughter of Toni Merlin and Harry Ratner. Following a brief marriage to Martin Lubner, she wed David Dretzin.
She served as the casting director for Follies, A Little Night Music, Into the Woods, and several other Stephen Sondheim musicals.