Van Maarsen studied in a regular school in Amsterdam until 1940, when Germany invaded the Netherlands.
In the Jewish school, she befriended many girls, including Anne Frank, Nanette Blitz, Sanne Ledermann and Hanneli Goslar.
After two and a half months in hiding, Anne composed a farewell letter to Van Maarsen in her diary, vowing her lifelong friendship.
Because she was Christian, Van Maarsen's mother was able to remove the "J" (Jew) signs from the family's ID cards.
Most of her father's side of the family perished in Nazi concentration camps including Auschwitz and Sobibor.
[6] As of 2023 (following the death of Hannah Pick-Goslar in October 2022), she is one of the last known surviving school friends of Anne Frank alongside Bergen Belsen survivor Nanette Blitz Konig.