[4] After graduating from high school, Cassidy attended West Virginia Wesleyan College in Buckhannon, where he was a member of the Alpha Sigma Phi fraternity.

According to Thomas "Duke " Miller, a TV/movie/celebrity expert, Cassidy also had a small role opposite George Peppard in one episode of the TV movie series Banacek.

Cassidy played a worker in an auto scrapyard who attempted to kill Banacek because the investigator traced him as part of the plot to steal a rare and valuable book.

[citation needed] In addition to The Addams Family, Cassidy found steady work in a variety of other television shows.

[9] He had a prominent role on NBC's The New Adventures of Huckleberry Finn as Injun Joe, the enemy of Tom Sawyer and Huck.

Cassidy did more work with Star Trek creator Gene Roddenberry in the early 1970s, playing Isaiah in the postapocalyptic drama pilots Genesis II and Planet Earth.

[14] Concurrent with his appearances on The Addams Family, Cassidy began doing character voices on a recurring basis for the Hanna-Barbera Studios, culminating in the role of Frankenstein Jr., in Frankenstein Jr. and The Impossibles series, and even reprising Lurch on several occasions for Hanna-Barbera productions (most notably for the Addams Family animated series in 1973–74).

Cassidy's final role was as King Thun of the Lion Men in the television animated feature film Flash Gordon: The Greatest Adventure of All.

That particular role was originally recorded shortly before Cassidy's death in 1979, until the decision was made to use the footage for a television series, The New Adventures of Flash Gordon.

After the series' conclusion, the original feature film and soundtrack were reassembled using Cassidy's performance and broadcast in prime time in 1982.

[15] Other film work includes Butch Cassidy and the Sundance Kid (1969), Mackenna's Gold (1969), The Limit (1972), Banacek (1972),Charcoal Black (1972), The Slams (1973), Thunder County (1974), Poor Pretty Eddie (1975), Harry and Walter Go to New York (1976), The Last Remake of Beau Geste (1977) and Goin' Coconuts (1978).

During that time, he also worked with Noel Marshall, the executive producer of Harrad Experiment, on the adventure-comedy film Roar (released two years after his death).

[19] Cassidy underwent surgery at St. Vincent Medical Center in Los Angeles to have a benign tumor removed from his heart.