The interest in Mn-based layered oxides for potassium-ion batteries (PIBs) cathodes stems primarily from their impressive capacity and economic viability. However, the presence of Mn³⁺ ions, which induce Jahn–Teller distortion, combined with sluggish ion diffusion kinetics, significantly undermines the cyclability and rate performance of the electrode, thereby limiting their practical application. Here, oxyfluorides are chosen instead of oxides to mitigate oxygen loss during prolonged cycling. Additionally, Li⁺ is incorporated into the composition to enhance the stability of the crystal lattice and accelerate the migration of K⁺. In situ X-ray diffraction (XRD) analysis reveals that the introduction of Li⁺ notably mitigates the detrimental P3–O3 phase transition within the high-voltage range. The designed K_0.45Li_0.045Mn_0.8Co_0.1Fe_0.05Ni_0.05O_1.95F_0.05 cathode exhibits high capacity of 123.7 mAh·g⁻¹ at 0.05 A·g⁻¹ and outstanding rate capability (83.0 mAh·g⁻¹ at 2 A·g⁻¹). Remarkably, the K-ion full battery exhibits an ultra-long cycle life (1000 cycles at 0.3 A·g⁻¹ with capacity retention of 75%), which fully demonstrates its great potential for practical application. This work offers a straightforward and efficient method for developing Mn-rich layered cathode materials, facilitating their practical implementation in advanced PIBs.