To develop high-safety and high-performance olivine LiMn1−xFexPO4/C cathodes for lithium-ion batteries, a scalable synthetic route is established by employing KMnO4 as the oxidant to prepare the MnFePO4·H2O precursor through a reverse-titration co-precipitation strategy. Nanosized spherical MnFePO4·H2O with a uniform elemental distribution and consistent valence states is successfully obtained, effectively suppressing Mn3+ disproportionation in an aqueous co-precipitation system. During lithiation, Li+ inserts into the initially vacant M1 sites, accompanied by the reduction of Mn3+/Fe3+ to Mn2+/Fe2+, thereby converting trivalent MnFePO4·H2O into olivine LiMn1−xFexPO4. The sand-milling spray-drying (SMSD) process further enables the formation of quasi-spherical secondary particles with a narrow particle-size distribution and a continuous highly graphitized carbon coating layer, in clear contrast to the irregular agglomerates and nonuniform carbon coatings generated by conventional ball-milling/stirring-drying treatment. As a result, the L-SMSD sample delivers a high discharge specific capacity of 153.5 mAh g−1 at 0.1C, an excellent rate capacity of 120.5 mAh g−1 at 5C, and outstanding cycling stability with a capacity retention of 95.15% after 450 cycles at 1C. Kinetic analysis further reveals that L-SMSD exhibits reduced polarization, enhanced Li+ diffusivity, and improved interfacial reaction kinetics. This work provides a practical route for the industrial-scale production of LiMn1−xFexPO4/C cathodes and is of great significance for the development of next-generation lithium-ion batteries.