<p>Polycrystalline diamond (PCD), as a composite of diamond particles and cobalt binder sintered under high temperature and pressure, is widely applied in many fields. EDM is recognized as a suitable machining method for PCD, although it commonly suffers from poor surface quality of the machined workpiece. This study aimed to investigate the machining of PCD with Cu-Ni electrodes under the ultrasound-assisted mixed-graphene powder EDM process. Specifically, the cobalt removal mechanism and the effect on the surface and properties of the machined workpieces were analyzed. Then, the effect of cobalt removal was verified by the friction wear experiments to explore the tribological properties of the workpieces. The results suggest that the cobalt content of the Cu-Ni electrode-machined workpiece was 63% and 40% lower than that of the untreated workpiece and the Cu electrode-machined workpiece, respectively. Compared with the Cu electrode, the Cu-Ni electrode-machined surface exhibited a porous structure composed of numerous pores, attributed to the presence of ultrasonic and Ni elements. Under lubricated friction conditions, the friction coefficient of the Cu-Ni electrode-machined workpiece was the lowest for a longer period of time. Under dry friction conditions, the surface of the Cu-Ni electrode-machined workpiece did not have any significant cracks compared with the surfaces of the untreated workpiece and the Cu electrode-machined workpiece. This contributed to the improved wear resistance of the machined surface.</p>