The austenite stability and the mechanical properties in a typical medium Mn grade steel, i.e., 5Mn steel, were investigated under a wide range of strain rates through the combination of experimental and theoretical methodologies. The obtained results indicate that austenite is more stable at a high strain rate, which is due to the suppression of the austenite to martensite transformation. This suppression is attributed to the increased stacking fault energy and the high deformation energy barrier. Moreover, the suppression of martensitic transformation also leads to the decrease in the ultimate tensile strength and the uniform elongation. Owing to the increase in an adiabatic heating temperature, an increase in the uniform elongation is acquired at a high strain rate. The obtained fundamental study results shed light on a wide application of the medium Mn steel under different strain rate conditions.