In this study, tensile tests for magnesium–manganese rare earth alloy (MnE21) were conducted with a WDW-300 high-temperature universal testing machine at different temperatures (300 °C~500 °C) and strain rates (1 × 10−4s−1~1 × 10−1s−1). The high temperature thermal deformation behavior, dynamic recrystallization, and texture of MnE21 magnesium alloy were analyzed by combining the constitutive equation, hot processing map, and electron backscatter diffraction (EBSD). The results show that the strain compensation equation can accurately predict the thermal deformation behavior. According to the hot processing map, the optimal processing regions were determined to be 350 °C, ε˙= 1 × 10−2s−1~ε˙= 1 × 10−4s−1, and 450–500 °C, ε ˙= 1 × 10−1s−1~ε˙= 1 × 10−4s−1. Based on the EBSD analysis, it was found that dynamic recrystallization of the alloy occurs above 350 °C, it was concluded that dynamic recrystallization was more adequate at 450 °C by analyzing the grain orientation and grain boundary difference orientation distribution. In addition, the texture index at different temperatures was also analyzed and it was found that the material showed a typical extrusion texture internally. During dynamic recrystallization, (01-11) [2-1-11], texture was produced.