John Joseph Edward Cassidy, known professionally as Jack Cassidy, was an American actor who achieved success in theater, cinema and television.

Cassidy was born in Queens, New York to William Cassidy, who was of Irish descent, and Charlotte Koehler, who was of German descent.

He achieved his greatest success as a musical performer on Broadway, winning a Tony Award for his role in She Loves Me . He also received Emmy Award nominations for his television performances in He & She (1967) and The Andersonville Trial (1970). On television he became a frequent guest, appearing in such programs as Bewitched, Get Smart, Columbo, Hawaii Five-O, Match Game and McCloud. He turned down the role of Ted Baxter in The Mary Tyler Moore Show, appearing instead as a guest star in a very memorable episode as Ted's highly competitive and equally egocentric brother, Hal. He also co-starred in the movie The Eiger Sanction with Clint Eastwood.

Cassidy was married twice. His first wife was actress Evelyn Ward, who is the mother of his son pop singer David Cassidy. After divorcing Evelyn Ward in 1956, he married actress Shirley Jones, whom he divorced in 1974. They had three sons: Shaun, Patrick, and Ryan Cassidy.

Cassidy died from burns received in a fire he caused by smoking in bed, in West Hollywood, California. In an eerie coincidence, he had starred in an episode of the TV series Night Gallery, called "The Last Laurel", in which he plays a villainous character who discovers the secret of astral projection, and bent on revenge against others, accidentally kills himself while sleeping.