This study examined Mn source effects on the electrochemical properties of Fe-substituted and Ni-substituted Li2MnO3 (Li1+x(Fe0.2Ni0.2Mn0.6)1−xO2, 0 < x < 1/3). Manganese sources of two kinds were selected: MnCl2·4H2O and KMnO4. Given the same preparation conditions, the chemical composition, transition metal distribution, and electrochemical properties of the two samples were compared. The sample obtained from KMnO4 exhibited better electrochemical performance, except for high-rate discharge characteristics, than that obtained from MnCl2. The origin of the different electrochemical performance was discussed to ascertain the unknown factor responsible for it. The full-cell performance was assessed using a single-layer laminate cell and mesocarbon microbead (MCMB) anode.