The high discharge potential of LiMnPO4, 4.1 V vs. Li/Li+, and its theoretical capacity of 170 mAh g−1 make it a promising candidate as a cathode material in lithium-ion batteries. But extremely low electronics conductivity, slow lithium diffusion kinetics, and the Jahn-Teller effect of Mn3+ limit the electrochemical performances of LiMnPO4. In this work, the pre-synthesized and defined FePO4·2H2O nanocrystallites are used as one of the raw materials to synthesize LiFe1−x Mn x PO4/C (x = 0.85, 0.75, 0.65) composites via solid-state reactions. The synthesized LiFe1−x Mn x PO4 samples show well-crystallized structures and have enhanced electrochemical properties. There exist two plateaus around 3.5 and 4.1 V on both of their charge and discharge curves. Among the samples, the Fe0.25Mn0.75PO4 one exhibits the longest high-voltage charge/discharge plateau at 4.10 V/4.05 V, and has an average discharge voltage of ∼3.78 V vs. Li/Li+ and a discharge capacity of ∼130 mAh g−1 at 0.05 C rate. For the Fe0.25Mn0.75PO4 sample, the noticeable improvement of its electrochemical performances is mainly attributed to iron substitution, the appropriate Mn/Fe ratio, and the well-ordered crystal structure forming by using FePO4·2H2O nanocrystallites as one of the raw materials.