Jean Thompson was one of five children born to Irene Celia Thompson (1919-2008) and Herbert Owens Thompson Sr. (deceased). She is of Norwegian heritage on her mother's side. She lived in Portsmouth, New Hampshire, before her civil-servant father moved the family to Guam in 1963. She attended the University of Guam at age 16, but after marrying a U.S. Navy lieutenant when she was 17, she left to work as a waitress and saleswoman on U.S. military bases throughout the Pacific Ocean to which her husband was posted. The marriage lasted six years. She moved to California and unsuccessfully attempted to find work in broadcast journalism. Frustrated, she joined an acting workshop and soon found roles on such TV series as Matt Houston, Fantasy Island and Alice.

She was married to radio personality Casey Kasem from 1980 until his death in 2014, and they had one child together, Liberty Jean Kasem.

In 1989, Casey Kasem purchased a house built in 1954, previously owned by developer Abraham M. Lurie, as a birthday present for her.

On October 1, 2013, Kerri Kasem, her siblings and their uncle, Casey's brother, protested outside Casey and Jean Kasem's home, saying Jean Kasem had been preventing contact with Casey Kasem for three months. Six days later, Julie Kasem and her husband, Dr. Jamil Aboulhosn, filed a conservatorship petition to place Casey Kasem under their care. The court denied their petition.

Casey Kasem died on June 15, 2014, aged 82. His widow was given her husband's body for funeral arrangements.