The naturally blonde-haired Kristen is best known for her role as Judy Robinson, the daughter of Professor John and Maureen Robinson, in the television series Lost in Space (1965–1968). Marta played the part of the space family's eldest child, a mature "20-something woman" near the age group of the space pilot, played by Mark Goddard. Her mature role allowed adult form-fitting fashions and hairstyles, as shown in publicity photos of the TV series.
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, Prof. & Mrs. Harold Soderquist, and was renamed Marta. She moved to Los Angeles, California, with her family in 1959 and is a graduate of Santa Monica High School.
She appeared in a 1961 episode ("Bang! You're Dead") of Alfred Hitchcock Presents alongside Billy Mumy who would later co-star with Kristen in Lost in Space. 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. Kristen also provided voice work for the 2009 animated theatrical short The Bolt Who Screwed Christmas which also included voice work from her Lost in Space co-stars Jonathan Harris, Bill Mumy and Angela Cartwright.
Kristen'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. She met her second husband, Kevin Kane, in 1974. She and her husband live in Santa Monica, California, with a pair of rescue dogs.