Mg alloys have high potential for use in lightweight structural components, especially in automotive applications, due to their low density and high specific strength. Many studies on Mg alloys for protection applications have been conducted, focusing on achieving high dynamic and spall strengths. However, thermal softening behavior is another important factor because highly localized shearing deformations can occur in such hexagonal metals. The deformation localization can lead to formation of adiabatic shear bands that result in macroscopic failure of the material, and understanding thermal softening is important for understanding shear band formation. In addition, however, the material can be highly anisotropic, and texture evolution provides a mode of geometric softening that can contribute to localization. Texture evolution in magnesium is often driven by twinning, and it is not clear how deformation twinning changes with temperature at high strain rates. We seek, therefore, to understand the evolution of deformation mechanisms in Mg alloys undergoing high rate deformations at high temperatures. Our investigations use high temperature Kolsky bar tests on Mg alloys, together with specimen recovery and microstructural evaluation, to understand these mechanisms. Our results show how the coupling of high strain rate and high temperature affect twinning, slip and the consequent dynamic mechanical properties of Mg alloys.