Kristen was born 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. In recent years she has starred in the 1998 television movie Lost in Space Forever and had a cameo role in the 1998 movie Lost in Space. She also appeared in the A&E Biography Jonathan Harris, Never Fear Smith Is Here in 2002.
Her 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 and an older sister whom she had never met, and in 1997 she discovered a brother residing in Australia. She met her second husband, Kevin Kane, in 1974; they remain married (as of 2006).