Catherine Willows was the assistant night shift supervisor to Grissom, until becoming supervisor of the CSI unit upon his departure. Born in Bozeman, Montana, Catherine Willows was raised by her single mother, a cocktail waitress and showgirl. Catherine failed to excel to her full potential in school, despite her intelligence and sharp mind.

Catherine left school and began work as an exotic dancer to support her boyfriend's career. She became interested in crime-solving when a regular at the dance club encouraged her to return to school. She graduated from West Las Vegas University with a degree in Medical Science. Catherine joined the CSI team as a lab technician and worked her way up to supervisor under Gil Grissom until taking over his role when he chose to leave CSI. Catherine has one daughter, Lindsey Willows (Kay Panabaker), and had a stormy relationship with late ex-husband Eddie Willows (Timothy Carhart); he was murdered in the episode Lady Heather's Box. Her relationship with her father, Sam Braun (Scott Wilson), also occasionally created conflicts in cases. In the episode Built to Kill, Braun is shot and dies in Catherine's arms.

After the events of season eleven, she was demoted back to asst. night shift supervisor under D.B. Russell (Ted Danson), later quitting CSI to join the FBI in season twelve.