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]]. [Her Finnish mother gave up little Birgit to save her the pain of growing up knowing her real parents were part of the Nazi regime or Third Reich.] BrigitBirgit spent her first years in an orphanage in Norway,<ref>{{Citation|last=Mad Bros Media|title=LOST IN SPACE 50th Anniversary Interview FEATURING MARK GODDARD & MARTA KRISTEN|date=August 15, 2015|url=https://www.youtube.com/watch?v=rXId4Sznav8&t=3m40s |archive-url=https://ghostarchive.org/varchive/youtube/20211213/rXId4Sznav8 |archive-date=2021-12-13 |url-status=live|location=4th GalactiCon, Seattle 2015|accessdate=September 8, 2017}}{{cbignore}}</ref> and was then adopted in 1949 by a couple from [[Detroit|Detroit, Michigan]], Harold Oliver Soderquist,<ref>{{Cite web|url=https://conservancy.umn.edu/bitstream/handle/11299/57553/1936-commencement.pdf?sequence=1|title=University of Minnesota COMMENCEMENT CONVOCATION WINTER QUARTER|page=45}}</ref> and his wife, Bertha,<ref name=":2">{{Cite web|url=http://martakristen.com/marta/magazine/elokuva16/index.htm|title=Elokuva-Aitta 16|website=martakristen.com|access-date=September 9, 2017}}</ref> who renamed her '''Martha Annalise Soderquist'''. Her adoptive father was a professor of education at [[Wayne State University|Wayne University]], Detroit.<ref name=":0">{{Cite web|url=http://martakristen.com/marta/magazine/tvguide/index.html|title=Norwegian Cinderella|website=martakristen.com|access-date=September 9, 2017}}</ref><ref>{{Cite journal|last=Soderquist|first=Harold|title=Personalistic Naturalism and the Ends of Education |date=January 1, 1954|journal=Educational Theory|language=en|volume=4|issue=1|pages=49–53|doi=10.1111/j.1741-5446.1954.tb01080.x|issn=1741-5446}}</ref> Kristen has a brother whom her parents adopted.<ref name=":1">{{Cite web|url=http://martakristen.com/marta/magazine/tvpicture/index.html|title=TV Picture Life - November 1966|website=martakristen.com|access-date=September 9, 2017}}</ref>
In 1959, Kristen moved to [[Los Angeles]], [[California]] when her father was on a sabbatical. She remained there, with a guardian, and is a graduate of [[Santa Monica High School]].<ref name=":0" />
