Cassidy's first film appearance was in "The Outfit" (1973). She appeared in a 1973 Smokey Bear public service announcement (PSA), and on such television series as "", "Starsky & Hutch" and "Taxi". She had a small role in "Stay Hungry" (1976), a film about bodybuilding that featured a young Arnold Schwarzenegger. Cassidy was considered for the role of Wonder Woman for a television series, but lost it to Lynda Carter. She co-starred in the film "Our Winning Season" (1978). Her first regular role was as sheriff's pilot Morgan Wainwright in the action-adventure series "240-Robert" (1979), although the series only lasted for two abbreviated seasons. Afterwards, Cassidy continued to appear in guest roles in series such as "Dallas" and "Falcon Crest", as well as a regular role on the short-lived sitcom "Buffalo Bill" (1983) (for which she earned a Golden Globe Award). She starred on the short-lived NBC television series "Code Name: Foxfire" (1985).