Born to Robert Smith (a newspaper editor) and Gail Dey (a nurse), Susan was a model before starring as Laurie Partridge in the television series The Partridge Family from 1970-1974. She was only 17 years old when she won the part, and had no previous acting experience.

While participating in The Partridge Family, striving to be as thin as possible, Dey developed an eating disorder, a form of orthorexia nervosa. While suffering from this disorder, she would only eat carrots. She denied having a problem until one day at a cast party at the beach. Co-star Danny Bonaduce exclaimed, "What's wrong with you?! Your skin is orange!" After this incident, and a struggle, Dey overcame her disorder and regained her normal health (and skin tone).

After the end of the television series, Dey strove to shed her wholesome image. She took movie roles where she was required to appear nude or semi-nude. The new image did not significantly help her career, however, and she remained little in demand for movie roles.

Dey seemed to be attempting a comeback with her appearance in 1981's sexy thriller Looker, but after this high-profile movie she nearly disappeared from the public eye until her starring role in the TV series L.A. Law as Grace Van Owen. She was critically hailed for her part in this role. She earned a Golden Globe Award as "Actress In A Leading Role — Drama Series" for this role in 1987. She was nominated four more times the four following years. She was also nominated for the Emmy Award for "Outstanding Lead Actress in a Drama Series" for this role in 1987, 1988 and 1989.

Dey has been married to her second husband, television producer Bernard Sofronski, since 1988. She has one child, a daughter, Sarah Hirshan (born 1978). Her daughter's father is her first husband, Lenny Hirshan (married 1976-1981). Susan Dey is a notable graduate of Fox Lane High School in Bedford, New York.

With the ending of LA Law, Dey has disappeared from public view. Reportedly, she has put her role as Laurie Partridge behind her and does not like to talk about her Partridge days.