His main sources of inspiration were Hendrik Werman and Piet Zwart, both groundbreaking Dutch typographers whose pioneering work abandoned the tenets of conventional symmetry. Sandberg also initially agreed with the "neue typographie" of Jan Tschichold and began to incorporate lowercase typographic characters and unjustified text in the majority of his work.

During World War II, Sandberg became a wartime hero as the only surviving member of a Dutch resistance group that in 1943 burned down Amsterdam's Municipal Office of Records in protest against the administration of the Nazi government.

After Europe's liberation in 1945, Sandberg became the director of the Stedelijk Museum. It was at the Stedelijk that he personally designed hundreds of its catalogs and posters, providing the museum with a unique brand and identity. As a designer, he produced innovative work characterized by the use of bold type, vivid colors, textured papers, and signature torn-paper forms.

From 1943 to 1945, while hiding from the Germans and working for the underground resistance, Sandberg produced the basis for Experimenta typographica, a series of print experiments in form, space, and tone presented in eighteen short, mostly handmade, books that were finally published in the 1950s and subsequently inspired his later work. These experiments included unjustified text settings and sentence fragments composed freely, with varying type weights and styles for visual interest or emphasis. They are void of symmetry and use bright colors, strong contrasts, and subtle tones for rhythm and pacing. Crisp sans serif typography is combined with large-scale, torn-paper, collaged letterforms with rough, irregular edges.

These sensitive explorations of compositional tone and space became enormously influential among a generation of graphic designers, as well as becoming the basis for many of Sandberg's later Stedelijk Museum catalogs, which were seen and then imitated around the world.

His body of work was a provocative marriage of the "neue typographie" combined with the expressive freedom of surrealism and the inevitable compromises of a wartime Europe.