Potassium sodium niobate ((K_0.5Na_0.5)NbO₃)-based lead-free piezoelectric ceramics are excellent ferroelectric materials and have been demonstrated to have many practical applications. Recent studies have revealed that chemical doping plays a crucial role in optimizing the electromechanical coupling properties of (K_0.5Na_0.5)NbO₃-based piezoelectric ceramics. In this paper, MnO₂ is doped into potassium niobate (KNbO₃) and (K_0.5Na_0.5)NbO₃ piezoelectric ceramics prepared by the conventional solid-state reaction method. The influences of doped Mn cation on KNbO₃ and (K_0.5Na_0.5)NbO₃ piezoelectric ceramics including microstructure and macroscopic electrical properties are systematically investigated. The doping effects of Mn cation on the KNbO₃ and (K_0.5Na_0.5)NbO₃ piezoelectric ceramics are significantly different from each other. For the Mn-doped KNbO₃ piezoelectric ceramics, the sizes of ferroelectric domains are reduced. Meanwhile, the diffused orthorhombic-tetragonal phase transition is observed, which is accompanied by reducing dielectric loss and Curie temperature, and broadening vibration peaks in Raman spectrum. It is known that the oxygen vacancy can be formed to compensate for the charges created by the acceptor doping of Mn into the B site of perovskite, and thus forming a defect dipole with the acceptor center. From the ferroelectric measurement, a double hysteresis loop (P-E curve) and a recoverable electric-field-induced strain due to the formation of defect dipole are observed. On the contrary, for the Mn-doped (K_0.5Na_0.5)NbO₃ piezoelectric ceramics, the sizes of ferroelectric domains are not reduced. Meanwhile, the Curie temperature and vibration peaks in Raman spectrum are not changed. A rectangular hysteresis loop (P-E curve) and an unrecoverable electric-field-induced strain are observed in the ferroelectric measurement. The difference between these systems might originate from the greater ionic disorder and lattice distortion in (K_0.5Na_0.5)NbO₃ piezoelectric ceramics. The difference in ionic radius between Na⁺ and K⁺ can affect the migration and distribution of oxygen vacancies, which makes it difficult to form stable defect dipoles in the Mn-doped (K_0.5Na_0.5)NbO₃ piezoelectric ceramics. The results will serve as an important reference for preparing high-performance (K_0.5Na_0.5)NbO₃-based piezoelectric ceramics via chemical doping.