A new synthesis route has been developed for LiMnyFe1-yPO4 powders. A significant improvement in electrode performance has been achieved by adding carbon black to the synthetic precursor. The carbon-containing LiMnyFe1-yPO4 was synthesized under various conditions and the performance of the cathodes was evaluated using coin cells. The samples were characterized by X-ray diffraction, particle-size distribution measurements, scanning electron microscope observations, and Brunauer, Emmett, and Teller surface area measurements. The addition of carbon black limited the particle size growth and enabled high electronic conductivity. Another advantage is simplification of electrode preparation, only needs the cathode powder to be mixed with binder. Even at the large Mn content of y = 0.75, a high capacity of 164 mAh/g has been achieved with an average discharge voltage of 3.63 V (595 Wh/kg) at room temperature. In addition, LiMnyFe1-yPO4 demonstrated excellent storage performance at elevated temperatures. The thermal stability of the charged cathode was evaluated by thermogravimetric and differential scanning calorimetric thermal analyses.