Lilleby closed on the same day Norway was attacked by Nazi Germany, but it did not remain closed for long. The Norwegian aluminium industry was of great strategic importance for the German government, which requested that Lilleby resume operation right away. Birger Solberg was placed in charge, because professor Pedersen had left with his family to Sweden. Solberg was dismissed the day that Pederson returned, undoubtedly related to his views on the occupation, which differed from Professor Pedersen's views: Pedersen was a supporter of Nazi Germany and wanted to collaborate with the occupation.
During the war, the plant was geared mostly towards aluminium, which was more important for the German war effort; however, many employees sabotaged the work in order to keep productivity low.