Smithers grew up in Woodland Hills, California. She played the character "Bailey Quarters" on the popular television series WKRP in Cincinnati. She first reached the public eye as a teenager, when she was featured on the cover of a 1966 issue of Newsweek, seated on a motorcycle. As a result of that exposure, she landed some commercial modeling work while pursuing her studies at the California Institute of the Arts.

As a teen, Ms. Smithers was involved in a car accident which left a permanent scar on her chin. To avoid another car that ran a stop sign, Jan swerved and hit a telephone pole. The impact smashed her face into the steering wheel, causing the injury. This scar cost her some roles, and according to Jan, made her feel insecure about her looks.

From 1987 through 1995 she was married to actor James Brolin. They had one daughter together. Although she had been living in Halifax, Nova Scotia, Canada and had retired from show business, she is now living in Ojai, California and is reported to be considering a return to acting.