He was best known for his novels set during the Second World War, during which he was an active member of the Dutch resistance. Keilson, having worked with traumatized orphans, mainly wrote about traumas induced by the war. His first novel was published in , but most of his works were published after the war. In , The New York Times 's Francine Prose described Keilson as "one of the world's greatest writers", notably honouring Keilson's achievements in the year in which he turned years old.
From to , Keilson studied pharmacology in Berlin, but due to the Nazi law prohibiting Jews from employment, Keilson was employed as a professional gym teacher in Jewish private schools, and occasionally made money as a musician. During this period, Keilson also met his first wife, graphologist Gertrud Manz In , the couple went into exile and fled to the Netherlands.
During his time here, Keilson wrote a few books in Dutch language, crediting himself under the pseudonym Benjamin Cooper. In , Keilson went into hiding and had to leave his pregnant wife behind. His wife gave birth to a daughter, Barbara, in the same year; she pretended the girl's father was a German officer to prevent prosecution.
Meanwhile, Keilson had moved in with a married couple in Delft , taking on a new identity as physician Dr. Van der Linden. During this time, the Dutch resistance asked him to pay visits to Jewish children that had been separated from their parents after they had gone into hiding. These experiences in particular formed the main inspiration for Keilson's later works.
Keilson reunited with his wife and daughter after the war. He and Gertrud were unable to marry before the war. In Germany they couldn't marry because of Keilson's Jewish origins. In the Netherlands it was not possible to marry for the Dutch law as German citizens.
