McDonald's motion Monday said "professional considerations require termination of the representation" and "good cause exists because withdrawal is mandatory" pursuant to an ethical rule.
Arpaio was charged with contempt for ignoring a court order in a racial profiling case to stop his immigration patrols. He has acknowledged prolonging his patrols, but says his defiance wasn't intentional.
If convicted, Arpaio faces up to six months in jail.
The 84-year-old Arpaio lost his bid for a seventh term as sheriff last November.