Westergren is the daughter of famous actors Håkan Westergren and Inga Tidblad. They moved to Nockeby, Stockholm, in 1931 and got married the same year. Westergren was born the following year on 16 May 1932. As a child, Westergren was a big admirer of her mother who was one of the biggest stars in Swedish theater. Westergren was raised with a nanny and has later said she wishes her busy parents would have spent more time with her while growing up.

Westergren was originally not planning on becoming an actress. Instead she decided to become a fashion designer and traveled to Paris to study. In between lessons at the designer institute, she walked the runway as a model at the fashion house Carven.