She contributed to the illegal newspaper De frie Danske, worked for the Danish-Swedish Refugee Service and joined the British Special Operations Executive (SOE).

[1] Bonnesen had already experienced the Nazis' racial policy when in 1935 she and her husband helped a Jewish couple in Denmark.

In early 1942, a friend brought her in touch with De frie Danske (Free Denmark), a resistance group which published an illegal paper with the same name.

In addition to contributing to the paper, she became involved in producing ration cards and false identity documents.

Her apartment on Tranegårdsvej in Hellerup was often used as a meeting place for resistance workers or for hiding wanted persons, including Mogens Hammer, the first SOE agent parachuted in Denmark.

She continued her illegal work with the radio specialist L.A. Duus Hansen who transmitted coded information to the SOE.

[1] In August 1944, while visiting the Danish-Swedish Refugee Service's illegal post office in central Copenhagen, Bonnesen was arrested and taken to the Gestapo headquarters in the Shell House.

[3][2] Since she had been identified by the Gestapo, Bonnesen left Denmark for neutral Sweden where she was employed as a secretary at the American consulate in Helsingborg.