Radio

Hagen began her show-business career in radio in the 1940s performing in Light of the World, Hollywood Story, and other programs.^[5] Using her birth name (Jean Verhagen), she played Betty Webster on Those Websters.^[6]

Stage

Hagen first appeared on Broadway in Swan Song. She acted in Another Part of the Forest, Ghosts, Born Yesterday,^[5][7] and The Traitor.^[8]

Film and television

Her film debut was as a comical femme fatale in the 1949 Spencer Tracy and Katharine Hepburn film Adam's Rib, directed by George Cukor. The Asphalt Jungle (1950) provided Hagen with her first starring role. Hagen played Doll Conover, a woman who sticks by criminal Dix's side until the bitter end. She appeared in the film noir Side Street (1950), playing a gangster's sincere but dim girlfriend. Hagen gave a memorable comic performance in Singin' in the Rain as the vain, spoiled, and talentless silent film star Lina Lamont. She received an Academy Award nomination for Best Supporting Actress for her performance.^[9]

By 1953, she had joined the cast of the television sitcom Make Room for Daddy. After she left the show, Marjorie Lord was cast as Danny's second wife and played opposite Thomas for the remainder of the series.

In 1957, Hagen co-starred in an episode of Alfred Hitchcock Presents titled "Enough Rope for Two", portraying a woman who accompanies two thieves trying to retrieve stolen money from a desert mine shaft. She then appeared as Elizabeth in the 1960 episode "Once Upon a Knight" on The DuPont Show with June Allyson; Also in 1960, Hagen was Marie Brandt on Wagon Train in the episode "The Marie Brandt Story"; and in 1963 Hagen portrayed Sarah Proctor on Wagon Train in the episode “The Sarah Proctor Story”. The following year, she guest-starred on The Andy Griffith Show in the episode "Andy and the Woman Speeder".

Although she made frequent guest appearances in television series, Hagen was unable to resume her film career in starring roles. Her health began to decline due to an alcohol problem and she spent many years hospitalized or under medical care in the 1960s.^[10] After appearing with Fred MacMurray in the Disney comedy The Shaggy Dog (1959), for the remainder of her career she played supporting roles, such as Marguerite LeHand, personal secretary to Franklin Delano Roosevelt, in Sunrise at Campobello (1960) and the friend of Bette Davis in Dead Ringer (1964).

Much later, 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. She made her final acting appearance the next year in the television movie Alexander: The Other Side of Dawn.