Hagen was born as Jean Shirley Verhagen in Chicago, Illinois, to Dutch immigrant Christian Verhagen (August 10 1890 - April 1983) and his Chicago-born wife, Marie (b. May 25 1896). The family moved to Elkhart, Indiana when she was 12 and she subsequently graduated from Elkhart High School. She studied drama and worked as a theater usherette before making her Broadway theatre debut in Another Part of the Forest in 1946. Her film debut was as a femme fatale in Adam's Rib (1949).
The Asphalt Jungle (1950) provided Hagen with her first starring role, and excellent reviews. She is arguably best remembered for her comic performance in Singin' in the Rain (1952). As the vain and talentless silent movie star Lina Lamont, Hagen received an Academy Award for Best Supporting Actress nomination. MGM failed to provide her with a quality follow up role to enable her to build on her growing popularity, and by 1953 she had joined the cast of Make Room for Daddy. As the first wife of Danny Thomas, Hagen received three Emmy Award nominations, but after three seasons she grew dissatisfied and left the series. Thomas, who also produced the show, didn't appreciate Jean's departing the successful series, and her character was killed off rather than recast. This was the first TV character to be killed off a family sitcom. Marjorie Lord was cast a year later as Dannys second wife and played against Thomas successfully for several seasons. Hagen later made a notable appearance as Frida Daniels in The Shaggy Dog (1959 film) starring with Fred MacMurray.
Although she made frequent guest appearances in various television series, she was unable to successfully resume her film career, and for the remainder of her career played supporting roles, such as the friend of Bette Davis in Dead Ringer (1964).
In the 1960s Hagen's health began to decline and she spent many years hospitalised or in care.
In 1976, she made a comeback of sorts playing character roles in episodes of the television series Starsky and Hutch and The Streets of San Francisco and made her final film appearance in a 1977 television movie (Alexander: The Other Side of Dawn) before her death from throat cancer, aged 54.
Hagen has a star on the Hollywood Walk of Fame for her contribution to television, at 1502 Vine Street.