Probably born in Tysnes in present-day Hordaland, Karen Mowat was the daughter of Admiral Axel Mowat (1592–1661) and Karen Knudsdatter Bildt (died 1662). Her father was the largest landowner in Western Norway. Following the death of her two brothers, she became sole heir to her parents' estate.
While her father was sceptical, Karen Mowat married Ludvig Rosenkrantz: a poor but highborn nobleman from Denmark. While Ludvig wanted to settle in Copenhagen, Karen had her will, establishing Rosendal in Kvinnherad in Hordaland as the family's main residence. They had four sons and five daughters including Justine Cathrine Rosenkrantz (1659–1746) who was a lady-in-waiting to Charlotte Amalie of Hesse-Kassel, queen consort of King Christian V of Denmark .
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