Abstract: High-entropy alloys (HEAs) exhibit pronounced strength–ductility synergy under extreme conditions such as high strain rates and cryogenic temperatures. However, existing crystal plasticity models, although capable of capturing macroscopic dynamic responses, rarely quantify strain-rate effects at the mechanistic level, and their critical parameters often lack explicit physical meaning. This limits the in-depth understanding of the physical mechanisms behind the excellent mechanical performance of HEAs. To address this issue, a physics-informed crystal plasticity finite element (CPFE) model was developed for equiatomic CrMnFeCoNi HEAs over a wide strain-rate range from 10⁻² to 10⁴ s⁻¹. Multiple deformation mechanisms were integrated into the model, and the model was implemented in a finite element environment. Based on a thermo-elasto-viscoplastic theory, strain-rate-sensitive crystallographic formulations were derived for the plastic deformation mechanisms, including dislocation slip, deformation twinning, and martensitic phase transformation. Two dislocation mechanisms, i.e., dislocation slip and viscous drag, were considered to describe rate-controlled dislocation activity. Discrete internal state variables, updated at each time step, were introduced to track the evolution of dislocation density and dislocation-motion resistance on individual slip systems. In addition, the model incorporated the temperature dependence of stacking fault energy (SFE), adiabatic temperature rise, and the temperature dependence of the Hall Petch relation. These components enabled quantitative characterization of the kinetic features of microstructural evolution and established a direct link between mesoscale microstructural evolution and macroscopic mechanical properties. Numerical simulations were performed for equiatomic CrMnFeCoNi HEAs across this wide strain-rate range. The model reasonably predicted the macroscopic plastic flow behavior and maintained consistent predictions of stress–strain responses and microstructural evolution. The results show that the nonlinear evolution of strength–ductility synergy under dynamic loading is jointly governed by strain-rate-dominated dislocation dynamics and SFE-regulated competition among dislocation slip, twinning, and transformation, which ultimately dictates the macroscopic mechanical responses of the alloy. By analyzing strain-rate sensitivity and work-hardening rate, the plastic stability during dynamic deformation was revealed. The simulation results validated the reliability of the model in capturing dynamic deformation mechanisms. It is concluded that the proposed framework provides a quantitative physical model for describing extreme deformation mechanisms in the Cantor (CrMnFeCoNi) alloy. It also offers a physically sound and transferable modeling framework for other face-centered cubic (FCC) HEAs and for the investigation and design of multi-principal element alloy systems with complex deformation mechanisms under dynamic deformation and temperature variation.