She earned a degree in theater at Northwestern University.
She provided the voice of Princess Odette in The Swan Princess and its sequels, The Swan Princess II: Escape from Castle Mountain and The Swan Princess: The Mystery of the Enchanted Kingdom, and the singing voice of Callisto for the Xena: Warrior Princess episode "The Bitter Suite". She also had guest starring roles in Airwolf Knight Rider and Wings. She appeared, briefly, as the college sweetheart of Billy Crystal in When Harry Met Sally. In 1996 she played Snow White in Coach episode "Grimmworld" as the girlfriend of Michael "Dauber" Daubinski (Bill Fagerbakke). Nicastro also had a minor role in Full House as Roxanne. She also played Lois "Old Lady" Scranton on an episode of Who's The Boss?. From September 1989 to May 1990 she appeared as singer Sasha Schmidt on Santa Barbara during one of its Daytime-Emmy-award-winning years.
On the stage, she created the role of Ariadne in the 1983 Broadway musical Merlin. She was the first Eponine in the second United States tour of Les Misérables in 1988.
Nicastro recorded four albums released on the Varese Sarabande label. Two albums, Toonful and Toonful Too feature songs from animated musicals, Reel Imagination features songs from family musicals, and On My Own features songs from contemporary Broadway musicals, including her version of On My Own. They feature Paul Goldberg on drums and percussion, Walt Fowler on trumpet, Jimmy Hoff on bass, and Lanny Meyers piano/arranger.