Arnold became a member of the German Democratic Party in the Weimar Republic and was a deputy for it in the Augsburg City Council from 1920.
He became co-owner of his father's cotton spinning and weaving mill Spinnerei und Weberei am Sparrenlech Kahn & Arnold, which had 940 employees in 1933 despite the world economic crisis.
Benno Arnold was deputy head of the Israelite religious community and board member of the Jewish old people's home.
The new owners were the Munich building contractor Fritz Schönmann and his wife Marion who were close to Adolf Hitler and Eva Braun.
On 27 January 2022, Holocaust Remembrance Day, at the request of the "Jewish Museum Augsburg Swabia" Foundation in Holzhausen am Ammersee, "Seehholzstraße 1-3" was unnamed "Anna und Benno Arnold-Platz".