The carbon-coated LiMn0.8Fe0.2PO4 (LMFP) mesocrystal, composed of ~40-nm-sized nanocrystallites, was designed to be favorable for the fast charge transport kinetics. The carbon-coated LMFP mesocrystal exhibited good electrochemical properties (i.e., high specific capacity and superior rate capability), ensuring that the LMFP mesocrystal is a proper model system to study the reaction mechanism upon the battery cycling. In order to investigate the electronic-structure effects of each transition metal (Mn and Fe) on the electrochemical performance, we performed synchrotron-based soft and hard x-ray absorption spectroscopy (sXAS and XAS), and quantitatively analyzed the changes of the transition-metal redox states in the carbon-coated LMFP electrodes during the electrochemical reaction. We believe that our comprehensive as well as complementary analyses using ex situ sXAS and in situ XAS can provide clear experimental evidence on the reaction mechanism of LiMn0.8Fe0.2PO4 electrodes during battery operation.