Dielectric ceramic materials have been actively studied for advanced pulsed power capacitor applications. Despite the good properties obtained in lead-based ceramics, lead-free counterparts are highly desired due to environmental regulations. This study revealed the potential of AgNbO3 to be a promising lead-free ceramic for energy storage applications. AgNbO3 ceramics fabricated using a conventional solid-state reaction method under an O2 atmosphere show a characteristic anti-ferroelectric (AFE) double hysteresis loop at an electric field of >130 kV cm-1, with a peak recoverable energy storage density (Wrec) of 1.6 J cm-3 at 140 kV cm-1. In addition, the incorporation of MnO2 into AgNbO3 can further increase Wrec, exceeding 2.3 J cm-3 at 150 kV cm-1 by the reduction of the remnant polarization, which is due to the enhanced AFE stability induced by the addition of MnO2. Of particular importance is that the 0.1 wt% MnO2-doped AgNbO3 ceramics were found to possess a good thermal stability with Wrec = 2.5-2.9 J cm-3 over a temperature range of 20-180 °C at 150 kV cm-1 and 1 Hz.