After gaining an M.A. in history, Söderhjelm spent 3 years in Paris, preparing her doctoral thesis under the supervision of Alphonse Aulard. This was a study of journalism during the French Revolution and it was published as Le Régime de la presse pendant la Révolution française. She was awarded a doctorate in 1900.
On the basis of this thesis, the university unanimously proposed to award her a lectureship. This appointment was delayed until 1906, because of political concern over her father and her brother. The Emperor was also concerned that if a woman became a lecturer in Finland, the same demand would be made in Russia.
In 1906 she finally became the first female lecturer in Finland. She stayed in this position until 1927. At this point, she became chair of General History at Åbo Akademi University, and thus the first female professor in Finland.
Söderhjelm worked as a journalist, writing a column for the newspaper Åbo Underrättelser. She also wrote novels, poetry, and a five-volume memoir. She co-wrote the screenplay for The Blizzard (1923), directed by Mauritz Stiller.
Söderhjelm was politically active. She smuggled journals into Finland from Sweden, and helped military volunteers to move from Sweden into Germany.