Kristen was born Birgit Annalisa Rusanen in Oslo, Norway, to a Finnish mother and a German soldier father who was killed during World War II. She was adopted in 1949 by an American couple from Detroit, Michigan and was renamed Marta. She moved to Los Angeles, California, with her family in 1959 and is a graduate of Santa Monica High School.
Her first success in acting was the role of Lorelei in the 1965 movie Beach Blanket Bingo. She would later star in Lost in Space and make numerous guest appearances on television shows. When her daughter was born in 1969 she began making television commercials and eventually appeared in more than 40.
Kirsten's personal life has revolved around discovering her roots, caring for her daughter, and now caring for her granddaughter. On a trip to Finland in 1969 she located her birth mother, Helmi Rusanen and an older sister whom she had never met, and in 1997 she discovered a brother residing in Australia.