[1][2] Born on 9 November 1859 in Ugilt near Hjørring, Classen was the daughter of the forester Henrich Wilhelm Claussen and his wife Ane Hedevig Cortsen.
[2] Claussen showed an early interest in art, sketching scenes of the surrounding countryside from the age of eight.
After her father died when she was 14, she was inspired to start writing poetry in a collection she titled Gisninger (Conjectures) in which she documented her religious and existential ideas.
[2] On returning from Vienna, she worked as a photographer in Aarhus for a short period but then went to Norway to study parapsychology.
In the mid-1890s, she returned to Denmark, where she turned to photography, opening first a studio in Brønderslev and then from 1897 in Skagen where she took portraits of both the town's residents and its summer visitors.