MnO x /C nanocomposite, in which MnO x nanocrystals (64 wt%, mainly MnO with Mn3O4 phase) 5–30 nm in size are formed in situ in the confined pores of a preformed mesoporous carbon, is prepared by a facile process. The morphology and structure of MnO x /C are characterized using N2 adsorption/desorption measurement, scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and thermogravimetric analysis (TGA). As an anode for Li-ion batteries (LIB), the MnO x /C delivers the theoretical capacity of MnO x (700–750 mAh g−1 at 100 mA g−1) with excellent rate capability. MnO x /C prelithiated is combined with a delithiated LiMn2O4 (LMO) cathode for a complete Li-ion battery cell in a charged state with a cathode limiting capacity. The LMO-MnO x /C cell exhibits reversible capacity of the LMO cathode, with the cycling stability and rate capability dependent upon those of the MnO x /C anode. An LMO-MnO x /C cell with anode loading in slight excess delivers reversible capacity of LMO without capacity fading for up to 245 cycles, and exhibits an appreciable rate capability at 2 C. The LMO-MnO x /C configuration can be developed as a reliable LIB, potentially offering low cost, high rate capability, safety, and reliability.