Hagen was born on August 3, 1923, in Chicago to Christian Verhagen, a Dutch immigrant, and his Chicago-born wife Marie. The family moved to Elkhart, Indiana when she was 12, and she graduated from Elkhart High School. She studied drama at Northwestern University, where she was a roommate of actress Patricia Neal. She graduated from Northwestern in 1945. She also worked as a theater usherette.

Jean Hagen married actor Tom Seidel (who originated the role of Dr. Sanderson in the play Harvey) on June 12, 1947, in Brentwood, California. The couple had two children, Christine Patricia Seidel and Aric Phillip Seidel. According to Lorraine LoBianco’s authoritative biography, Seidel, in his attempt to stop his wife from drinking, divorced her and gained custody of the children. It did not work; Hagen’s alcoholism only worsened, finally becoming so severe by 1968 that she was hospitalized and lapsed into a coma at UCLA Medical Center. She managed to survive the ordeal and her daughter Christine said that after she emerged from the coma, Hagen never drank again.

By 1953, she had joined the cast of the television sitcom Make Room for Daddy. For her portrayal as the first wife of Danny Thomas, Hagen received three Emmy Award nominations but after three seasons, she grew dissatisfied with the role and left the series. Thomas, who also produced the show, reportedly did not appreciate Hagen’s departure and her character was killed off rather than recast, the first TV character to be killed off in a family sitcom. Marjorie Lord was cast a year later as Danny’s second wife and played opposite Thomas successfully 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; the following year, she guest-starred on The Andy Griffith Show in the episode “Andy and the Woman Speeder”.

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.

Hagen died, twenty-six days after her 54th birthday, of esophageal cancer on August 29, 1977, at the Motion Picture & Television Country House and Hospital, and was buried in Chapel of the Pines Crematory.