The evolution of deformation twins and mechanical properties of medium manganese high-carbon steel in the deformation temperature range of 85–270 °C were studied via an AG-X PLUS 100 kN universal testing machine equipped with heating furnace, as well as with a field emission scanning electron microscope (FESEM), X-ray diffraction (XRD), and high-resolution transmission electron microscope (HRTEM). The results show that tensile strength of medium manganese high-carbon steel first increases and then decreases; the uniform elongation increases with the increase in deformation temperature. The best mechanical properties of experimental steel appear at a deformation temperature of 180 °C. The morphology of the twin boundaries changes from flat to bending, and the bending degree of the twin boundary increases with increasing deformation temperature. There are three causes leading to twin boundary bending at low deformation temperature, while only one causes twin boundary bending at high deformation temperature. The main reasons for twin boundary bending at low temperature are the formation of twin nucleus, the reaction of dislocation and stacking faults, and the conversion of coherent and incoherent lattices; that of sample heated to 270 °C is only the reaction between dislocation and stacking faults. In addition, a strength calculation model was developed to calculate the strength of experimental steel at different deformation temperatures.