In 1868, she supported herself as a teacher at the Frøknerne Villemoes-Qvistgaards Institut in Copenhagen and worked as a governess in the provinces a couple of years. In 1874, she corresponded with the Swedish female physician
Charlotte Yhlen, who advised her to contact the Danish parliamentarian C. E. Fenger, who was known to support women's rights and who had previously supported the first female Danish telegrapher,
Mathilde Fibiger. Fenger forwarded her apply to study medicine at the university to the ministry of education. In 1875, a royal decree issued the reform to allow women access to university education. In 1877, she and
Johanne Gleerup became the first two female university students in Denmark. She was given a small allowance from the
Dansk Kvindesamfund to make it economically possible for her to study. She graduated as a medical doctor and physician in 1885. She established herself as a medical practitioner in Copenhagen. She had planned to specialize in gynecology, but the only Danish gynecologist at the time, F. Howitz, would not accept her. However, in 1906 she was appointed communal specialist in venereal diseases and engaged in the rights of prostitutes. Nielsine Nielsen was active in the women's movement through the
Dansk Kvindesamfund (DK). However, critical of the much too careful DK, she became engaged in more radical women's groups. In 1893–1898, she was the chairperson of the women's suffrage association
Kvindevalgretsforeningen (KVF). In 1904, she and
Louise Nørlund, Birgitte Berg Nielsen and Alvilda Harbou Hoff became the first female members of the liberal party. In 1907, she was one of the co-founders of the
Landsforbundet for Kvinders Valgret. ==Legacy==