She began working as a model in the 1960s and made her professional acting debut in 1973, appearing in the thriller films The Laughing Policeman and The Outfit.

[2] She later starred in films The Fourth Protocol (1987), Who Framed Roger Rabbit (1988) for which she received a Saturn Award nomination, The Package (1989), Where the Heart Is (1990), Don't Tell Mom the Babysitter's Dead (1991), Vampire in Brooklyn (1995), and Ghosts of Mars (2001).

Cassidy also starred in a number of made-for-television movies and miniseries, include Hollywood Wives (1983), Invitation to Hell (1984), Barbarians at the Gate (1993) and The Tommyknockers (1993).

[9] During her time there, she married Kennard C. Kobrin in 1963, a doctor in residency, and found work as a fashion model.

They moved to San Francisco, where her husband set up a psychiatric practice while Cassidy continued modeling.

[2] She appeared in a 1973 Smokey Bear public service announcement (PSA),[10] and on such television series as Mission: Impossible, Starsky & Hutch and Taxi.

She played the ex-wife of James Garner's lead character in the television movie, The Rockford Files: I Still Love L.A. (1994).

Her other screen credits from this era include Barbarians at the Gate (1993), the miniseries adaptation of Stephen King's The Tommyknockers (1993), and Wes Craven's Vampire in Brooklyn (1995).

For the Final Cut, Cassidy's head was digitally transposed onto footage of the stunt performer, making the death scene fit continuity.

[13] In the second season of the NBC series Heroes, she is seen in a photo of the 12 senior members of the show's mysterious company.