He worked as a jailer during Oswald's regime.
When the Third Doctor was taken to prison prior to being executed, he met Arnold and convinced him to help him escape, but they were caught. When Oswald was overthrown, Arnold was released and the Doctor invited him to join him in the TARDIS. (COMIC: Children of the Evil Eye)
After they dropped off Oswald on another planet, the Doctor and Arnold tried to help a fleet of ships that appeared to be heading directly into a supernova. This turned out to be an illusion. The fleet was crewed by the Spidrons. After escaping the Spidrons' city, the Doctor and Arnold encountered the primitive, ape-like natives who also lived on the Spidron world, and worked with them to attack and destroy the Spidrons. (COMIC: Nova)
Behind the scenes