Studies on the temperature dependence of microstructure evolution for four models, namely single crystal and polycrystalline FeCoNiCrMn high-entropy alloys as well as single crystal and polycrystalline Ni, help elucidate the microscopic mechanisms of plastic deformation. Based on the nanoindentation method, this work focuses on comparative analysis of the effects of temperature variation from an extremely low temperature (1 K) to an extremely high temperature (1300 K) on the plastic deformation microstructure evolution, atomic displacement magnitude, shear deformation, total dislocation length, and von Mises stress distribution of single crystal and polycrystalline FeCoNiCrMn high-entropy alloys, together with single crystal and polycrystalline Ni. The results show that when the temperature rises from 1 K to 1300 K, the maximum load decreases by 37.69%, 29.10%, 55.06%, and 15.64% for single crystal FeCoNiCrMn high-entropy alloy, single crystal Ni, polycrystalline FeCoNiCrMn high-entropy alloy, and polycrystalline Ni, respectively. Defects exist at grain boundaries in polycrystalline FeCoNiCrMn high-entropy alloy and polycrystalline Ni; high stress concentration at grain boundaries induces dislocation nucleation and proliferation at grain boundary edges. The plastic deformation microstructure evolution of the four models under nanoindentation loading is dominated by the synergistic effect of stacking faults, twins, amorphous phases, and phase transformation: plastic deformation of single crystal FeCoNiCrMn high-entropy alloy generates bulge-like structures that restrain dislocation slip and propagation; prominent dislocation slip is observed in plastically deformed single-crystal Ni. Compared with polycrystalline Ni, polycrystalline FeCoNiCrMn high-entropy alloy forms thicker amorphous layers and exhibits shorter total dislocation length during plastic deformation, and this trend is weakly affected by temperature change from cryogenic to high temperature. Grain boundary migration and expansion occur in both polycrystalline FeCoNiCrMn high-entropy alloy and polycrystalline Ni during plastic deformation, which facilitates the formation of finer nanocrystalline structures. These findings provide important theoretical references for developing high-entropy alloy systems with outstanding strength-toughness synergy.