The phase controllable synthesis of carbon-coated monoclinic LiMnBO3 (m-LiMnBO3/C) and hexagonal LiMnBO3 (h-LiMnBO3/C) composites has been achieved via an in-situ carbothermal solid state synthesis approach only through the modulation of the reaction temperature. The h-LiMnBO3/C particles keep high cycle stability and retain 86.5% of the initial discharge capacity (90.7 mAh g−1) after 40 cycles at 0.05 C (11 mA g−1) within 1.25–4.80 V, while the m-LiMnBO3/C composites display a first discharge capacity of 107 mAh g−1 and a capacity retention rate of 80.8% after 40 cycles. The rate performance of m-LiMnBO3/C has been tested for the first time, which has a capacity of 74.4 mAh g−1 at the discharge rate of 0.1 C (22 mA g−1). The enhanced electrochemical performance might be largely attributed to the uniformly coated carbon layers and the reduced particle size of the as-obtained products. The electrochemical impedance spectroscopy (EIS) analysis reveals that the smaller charge transfer resistance (R ct) and higher electronic conductivity of the LiMnBO3/C composites were obtained at 55 °C than those at 25 °C. The as-obtained LiMnBO3/C composites with controllable phases and high performances enable their promising applications as cathode materials for lithium-ion rechargeable batteries.