Lead-free (1 − x)SrTiO3–xBi(Mg0.5Hf0.5)O3 (0.05 ≤ x ≤ 0.4) ceramics were prepared by high-temperature solid-state sintering method. The microstructure, dielectric properties, and energy storage performance were investigated. The diffused phase transition was observed in samples with x ≥ 0.1, showing typical relaxor characteristic. In particular, 0.8ST-0.2BMH composition exhibits stable dielectric permittivity with variation below 15% over temperature range of − 100 °C to 185 °C, superior to that of undoped SrTiO3. The calculated Weibull characteristic breakdown strength is 480 kV/cm for 0.8ST-0.2BMH ceramic, and a large energy density of 4.1 J/cm3 with high energy efficiency of 92% is obtained at electric field of 470 kV/cm. Of particular significance is that the energy efficiency remains above 91% at elevated temperature of 180 °C. In addition, fast discharge time (~ 0.88 µs) with high-power density of 2.8 MW/cm3 is achieved in 0.8ST-0.2BMH ceramic. All the above results indicate that the 0.8ST-0.2BMH relaxor dielectric is a promising lead-free ceramic for high-temperature energy storage applications.