The kinetic processes during delithiation/lithiation of Li x Mn0.8Fe0.2PO4 are thoroughly investigated through operando x-ray diffraction and in situ electrochemical impedance spectroscopy combined with galvanostatic intermittent titration technique (GITT), by which new insights on the phase propagation and sluggish kinetics of LiMn0.8Fe0.2PO4 (LMFP) cathode materials are elaborated. In situ analyses on the solvothermally synthesized carbon-coated LMFP mesocrystals reveal that the phase-propagation mechanisms differ during delithiation/lithiation processes, and the sluggish kinetics of LMFP followed by the limitation of achievable (dis)charge capacities originate from the poor apparent Li+ diffusivity, which is triggered by Mn redox reaction. Based on the in-depth characterization of the reaction kinetics in LMFP mesocrystals, our work provides fundamental understanding to design high-performance Mn-based olivine cathodes.