Layered-type Mn-based cathode materials are widely known as cathode materials for potassium-ion batteries (KIBs) due to their high specific capacity resulting from lower molar mass. However, it suffers from a serious problem due to the 1) strong Jahn-teller effect of Mn³⁺ ion and 2) large ionic radius of K⁺ ion, which causes not only irreversible structural change during charge/discharge but also poor electrochemical properties. Thus, we suggested P2-K_0.48[Li_0.1Mn_0.9]O₂ (P2-KLMO) as a promising Mn-based cathode material for KIBs with improved structural stability and electrochemical properties by substituting Li ions within the transition metal layer. Through the first-principles calculation, we predicted that the P2-KLMO was significantly suppressed structural change through Li substitution, thereby proving excellent electrochemical performance. Based on these results, at 13 mA g^-1 current density, P2-KLMO delivered a specific capacity of ~124.3 mAh g^-1 in the voltage range of 1.5-4.1 V (vs. K⁺/K), corresponding 0.46 mol of K⁺ de/intercalation in the structure during charge/discharge. Moreover, the combined analysis such as first-principles calculation and various ex-situ techniques were performed to verify the reversible reaction mechanism during charge/discharge. Our findings will help pave the way for designing layered-type Mn-based cathode materials with superior electrochemical performance for KIBs.