The LiMgxMn1-xPO4/C (x=0, 0.02, 0.04, 0.12) composite cathode was successfully prepared by a combination of spray pyrolysis and wet ballmilling with heat-treatment. The composite cathode had narrow particle size distribution with an average particle size of 99 nm. The Mg doping on the Mn site led to the electrochemical performance enhancement of the composite cathode, which was confirmed by cyclic voltammetry, ac impedance spectroscopy, and charge-discharge tests. The Mg-doped composite cathode exhibited a high discharge capacity in lithium cell, which remarkably increased with an increase in the charge cutoff voltage under galvanostatic charge-discharge. The LiMg0.04Mn0.96PO4/C cathode exhibited a discharge capacity of 154 mAh g(-1) (above 93% of the theoretical value) at 0.1C when charge-discharged galvanostatically to 4.9 V. Along with enhanced discharge capacity, the cell exhibited a good rate capability under the galvanostatic charge-discharge. Under the trickle mode conditions, the cell exhibited discharge capacities of 154, 136, 106, and 74 mAh g(-1) at 0.05, 0.1, 1, and 5C, respectively.