The validity of Mn element on enhanced energy storage performance and fatigue resistance of Mn-doped 0.7Na0.5Bi0.5TiO3–0.3Sr0.7Bi0.2TiO3 lead-free ferroelectric ceramics (BNT–BST–xMn) is certified by doping. The effects of Mn modification on the dielectric behavior, ferroelectric, energy storage properties, and AC impedance are comprehensively investigated. It is found that the average grain size of the ceramics modified by Mn additions is reduced slightly. Moreover, the relaxor properties are evidently enhanced with the increased Mn content. The AC impedance spectra can even better clarify the dielectric response and relaxor behavior. The results suggest that both of the dielectric response and relaxor behavior are determined by defects especially concentration of the oxygen vacancy. The superior energy storage properties are realized at x = 0.05 with an energy storage density (Wrec) of 1.33 J/cm3 as well as energy storage efficiency (η) of 86.2% at 100 kV/cm, accompanied with a superior thermal stability. BNT–BST–5Mn ceramics can maintain a stable energy storage performance within 106 fatigue cycles, indicating an excellent fatigue resistance.