Karl Arnold was born in Germany in 1883. After training as a painter his cartoons began appearing in German magazines such as Die Jugend and the Munchener Illustrierte Presse. In 1907 he started contributing to Simplicissimus. He soon became a staff artist and a shareholder in the magazine.
Arnold, like most of those working for Simplicissimus, was opposed to the foreign policy of the German government before the outbreak of the First World War. However, once fighting began, Simplicissimus gave its full support to the war effort. Ludwig Thoma, the editor, later reported: "All of us had supported peace. With no cautious reservations we had denounced the personal rule and all its harmful manifestations. But once the war was there nothing mattered but our own country."
After the war Simplicissimus led the campaign against the Versailles Treaty. In the 1920s Simplicissimus defended the Weimar Republic against threats from the revolutionary left and right-wing nationalism. It strongly opposed Adolf Hitler and the right-wing press accused Simplicissimus of being under the control of the Jews.
Whereas some German artists such as George Grosz and John Heartfied portrayed Adolf Hitler as dangerous and immoral, Arnold made him look silly. One active member of the Nazi Party once asked Arnold: "Do we really look the way you draw us?" and he replied, "Yes, and if some knew how they look, they wouldn't dare show themselves in the street."
When the Nazis gained power in 1933 stormtroopers arrived at the offices of Simplicissimus and some of the cartoonists, such as Thomas Heine and Walter Trier left the country but Arnold carried on working at the magazine and during the Second World War produced pro-Hitler cartoons.