A novel carbonophosphate, Na3MnPO4CO3, was synthesized as a cathode material using a mechanical ball milling method with starting materials of MnCO3 and Na3PO4 without washing or drying. Duo to the formation of nano-size particles and good dispersion of the obtained Na3MnPO4CO3, the initial discharge capacity in an organic electrolyte of 1 M NaPF6/ethylene carbonate (EC): dimethyl carbonate (DMC) (1:1 v/v) was 135 mAh∙g−1 and 116 mAh∙g−1 at 1/30 C and 1/10 C, respectively. We also investigated the cathode properties of Na3MnPO4CO3 in an aqueous electrolyte of 17 m NaClO4. This is the first investigation of the electrochemical performance of Na3MnPO4CO3 with aqueous electrolyte. Na3MnPO4CO3 achieved a discharge capacity as large as 134 mAh g−1 even at a high current density of 2 mA cm−2 (0.5 C), because of the high ionic conductivity of the aqueous electrolyte of 17 m NaClO4.