[3] On television, he became a frequent guest star, appearing in such programs as The Alfred Hitchcock Hour, Gunsmoke, Bewitched, Get Smart, That Girl, Hawaii Five-O, Cannon, Match Game, McCloud, and Barnaby Jones for an episode titled "Murder in the Doll's House" (1973).

Cassidy also co-starred as an informer in the movie The Eiger Sanction with Clint Eastwood and provided the voice of Bob Cratchit for the pioneering animated television special Mister Magoo's Christmas Carol.

[3] His frequent professional persona was an urbane, witty, confident egotist with a dramatic flair, much in the manner of Broadway actor Frank Fay.

[citation needed] The role of the vain, shallow, buffoon-like newsman Ted Baxter on TV's The Mary Tyler Moore Show (1970–1977) was reportedly written with Cassidy in mind.

[4] Cassidy had played a similar buffoonish character in the 1967–1968 sitcom He & She, but he turned down the role, feeling that it was not right for him; the part went to Ted Knight.

Cassidy later appeared as a guest star in a 1971 episode[5] as Ted's highly competitive and equally egotistical brother Hal.

In his 1994 autobiography, C'Mon, Get Happy, Cassidy's eldest son David wrote that he became increasingly concerned about his father in the last years of his life.

[7] Cassidy's second wife, Shirley Jones, described a similar incident when she found him sitting naked in a corner of their house, reading a book.

[12] On December 11, 1976, Cassidy invited his ex-wife, Shirley Jones, to his home, an apartment in West Hollywood, California, for drinks, but she declined.