In 1992, paying special attention to aspects relating to gender, she earned a doctorate with Viljens styrke, a biography of the pioneering feminist and educator Natalie Zahle.
From 1991 to 1998, she was chief archivist and head of the Manuscript Department at the Royal Danish Library where she was responsible for the registration of literary sources.
After gaining the status of professor at the University of Southern Denmark in 1999, in 2000 she was appointed senior research fellow at the Danish National Archives.
[6] In connection with her biography of Natalie Zahle, Possing commented: "I came across her archive while trying to find out why the distribution of power and gender had been the way it was in Denmark.
I discovered that all the women who were pioneers in Denmark came from her school — all those who made their mark in science, politics, education and business.