Semmingsen was born in Hamar, as the daughter of Lieutenant Colonel and farmer Peter Thorvald Gaustad (1878–1949) and his wife Gudvor, née Todderud (1884–1956).
[3] She published Standssamfunnets oppløsning i Norge in 1954, and Husmannsminner (1960) was the result of a project at the Norwegian Museum of Cultural History.
She retired in 1978,[3] but long after this she has been considered an authority on the Norwegian immigration to the United States, together with Odd Lovoll.
[4] Lovoll has cited Semmingsen as his primary inspiration,[5] and called her body of work a "national treasure".
[6] Semmingsen was a member of the Norwegian Academy of Science and Letters from 1952, and held honorary degrees at the Uppsala University and St. Olaf College.