He was born in Richmond Hill, New York, the son of Charlotte (née Koehler) and William Cassidy. His father, who was engineer at the Long Island Rail Road, was of Irish descent and his mother was of German ancestry.  Cassidy achieved his greatest success as a musical performer on Broadway, appearing in Alive and Kicking, Wish You Were Here, Shangri-La, Maggie Flynn, Fade Out – Fade In, It’s a Bird…It’s a Plane…It’s Superman, and She Loves Me, for which he won a Tony Award. He also received Emmy Award nominations for his television performances in He & She and The Andersonville Trial.  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, Match Game and McCloud and three times as a murderer on Columbo, including the acclaimed “Murder By the Book” episode directed by Steven Spielberg. Because that episode was the first one of the first season, Cassidy was the first murder suspect of the series, excluding two previous pilot episodes.

He co-starred with Ronnie Schell in a TV revival of Hellzapoppin’. Cassidy also co-starred as a killer in the movie The Eiger Sanction with Clint Eastwood and provided the voice of Bob Cratchit for the pioneering animated TV special Mr. Magoo’s Christmas Carol.  His frequent professional persona was that of an urbane, super-confident egotist with a dramatic flair, much in the manner of Broadway actor Frank Fay. Cassidy perfected this character to such an extent that he was cast as John Barrymore in the feature film W.C. Fields and Me.  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. Although Cassidy had played a similar buffoonish character in the 1967-68 sitcom He & She, 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 as Ted’s highly competitive and equally egotistical brother, Hal.

Cassidy was married twice. His first wife was actress Evelyn Ward. Together they had a son, David, who later became a teen idol. After divorcing in 1956, Cassidy married singer and actress Shirley Jones. Cassidy and Jones had three sons, Shaun, Patrick, and Ryan. Cassidy’s eldest son David later starred with Jones in the musical sitcom The Partridge Family. Jones and Cassidy divorced in 1974.  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. Cassidy suffered from bipolar disorder and was an alcoholic, who was displaying increasingly erratic behavior. In 1974, his neighbors were shocked to see him watering his front lawn naked in the middle of the afternoon. Cassidy’s second wife Shirley Jones described a similar incident when she found him sitting naked in a corner, reading a book. Jones said to him that they had to get ready to do a show, and he calmly looked up and said, “I know now that I’m Christ”. In December 1974, Cassidy was hospitalized in a psychiatric facility for 48 hours. At that time, Jones found out that he had been previously diagnosed with bipolar disorder.

David Cassidy also claimed that his father was bisexual, citing attributed personal accounts and reports, both anecdotal and published, of his father’s same-sex affairs, a fact neither he nor his siblings discovered until after his death. In her 2013 memoir, Shirley Jones confirms that Cassidy had many same-sex affairs, including one with Cole Porter.  By 1976, Cassidy was living alone in a penthouse apartment in West Hollywood.  According to ex-wife Shirley Jones, on December 11, 1976, Cassidy asked her over for drinks but she declined his invitation. He then ate dinner alone at an Italian restaurant. Cassidy returned to his apartment by himself, by which time he was drunk. In the early morning hours of December 12, Cassidy lit a cigarette and fell asleep on his Naugahyde couch. He then dropped the cigarette, which ignited the couch. The flames quickly spread throughout the apartment and the building. A charred body, burned beyond recognition, was found in the doorway of the apartment. As Cassidy’s car was missing (it was later returned by a friend who had borrowed it), his family hoped that he had traveled to Palm Springs, which were his intended plans for the following day but the body was then positively identified as Cassidy’s by dental records and by a signet ring that he wore, bearing the Cassidy family crest. His remains were cremated and scattered on the Pacific Ocean.