Returning to Amsterdam, Sandberg started work as a graphic designer, utilizing his printing skills and Neurath's Isotype system.

During the Second World War, Sandberg was active in the Dutch resistance movement, preparing forged documents for Jews and others wanted by the Gestapo.

[4] Within a short time, the Nazis began to expose the false documents by comparing the names with those in the local population registry.

[6] For his participation in the resistance movement and helping to save the lives of Jews during the war, Sandberg was recognized as Righteous Among The Nations on 26 November 1968.

During his tenure as curator, Sandberg expanded the museum and developed new exhibition techniques, for which he gained international renown.