A family of lithium mixed transition-metal pyrophosphates Li2Fe1−yMnyP2O7/C (0≤y≤1) has been prepared by mechanochemically assisted solid state synthesis using a high-energy planetary mill AGO-2. The samples were studied by X-ray powder diffraction (XRD) including Rietveld refinement, Fourier transform infrared spectroscopy (FTIR), Mössbauer spectroscopy, scanning electron microscopy (SEM), 6Li nuclear magnetic resonance spectroscopy (NMR), galvanostatic cycling, and galvanostatic intermittent titration technique (GITT). All samples were refined in a monoclinic framework (space group P21/c) with M ions occupying MO6 and MO5 sites and Li ions occupying LiO4 and LiO5 sites. The cell volume of Li2Fe1−yMnyP2O7 increases vs. Mn content with a slight deviation from the linearity. M2+ ion fractions in different coordination polyhedra and structural disordering among the Li and M sites in the as-prepared pyrophosphates were studied in detail by a multi-technique approach. According to galvanostatic cycling, only the Fe2+/Fe3+ redox couple operates upon the cycling of Li2Fe1−yMnyP2O7 in the 2.0-4.3V range, while the Mn2+/Mn3+ redox pair becomes active at higher voltage (∼4.8V). The Li2Fe0.75Mn0.25P2O7/C sample shows the highest discharge capacity of 87mAhg−1, close to the theoretical capacity delivered by the Fe2+/Fe3+ redox couple. A single-phase mechanism of lithium deintercalation was confirmed. Lithium diffusion coefficient D Li+ during the charge-discharge processes of Li2Fe0.75Mn0.25P2O7/C is within the range from 10−13 to 10−15 cm2 s−1 which is slightly higher than that of Li2FeP2O7/C.