Dielectric capacitors with high energy storage properties are key enablers for potential applications. We report SrTi_0.985(Zn_1/3Nb_2/3)_0.015O₃–x wt%ZnNb₂O₆ ceramics with high breakdown strength and energy storage performance by synergy manipulation. The structures, energy storage and dielectric properties of the ceramics are systematically investigated. Introduction of ZnNb₂O₆ mediates the contradiction between permittivity and breakdown strength. As a result, a high breakdown strength of 422 kV cm⁻¹ and an excellent energy storage density of 2.35 J cm⁻³ are achieved in x = 4.5 ceramics, which also exhibit fast discharge features (τ_0.9 < 1.5 μs), good thermal stability (25–150 °C) and outstanding cyclic characteristics (up to 5 × 10⁵ times). Further results indicate that ZnNb₂O₆ additives partly dissolve into perovskite lattices to promote the formation of superlattice structures, improving the local polarization, and partly disperse around grain boundary regions to inhibit grain growth and relieve the direct damage from a strong electric field to grains, increasing the breakdown strength. This strategy should be generalizable for designing high performance dielectrics and other novel composite materials that benefit from synergistic effect manipulation.