Electric-field control of ferromagnetism is demonstrated in a mechanically flexible solid-state system through a proton-induced redox reaction at room temperature. Protons transported through a perfluorosulfonic acid (PFSA) proton exchange membrane (PEM) trigger the reduction of CoO by reacting with lattice oxygen, which enables reversible switching between paramagnetic and ferromagnetic states starting at voltages below 10 V. The proton supply is sustained by ambient humidity and water splitting at a Pt thin film counter electrode. Due to its flexibility, the device architecture—a sandwich of CoO and Pt thin films integrated with the polymeric PEM—retains full magneto-ionic functionality under mechanical bending. Three CoO films, synthesised via reactive sputtering under varying conditions, are compared: amorphous, crystalline, and mixed-phase CoO/Co. While the amorphous film exhibits weak response, both the crystalline and mixed-phase films show pronounced electric-field-dependent magnetic switching, highlighting the critical role of microstructure in magneto-ionic performance.