This study investigates the application of laser shock peening (LSP) as a surface modification technique to enhance the mechanical performance of high-entropy alloys (HEAs), focusing on the CrMnFeCoNi Cantor alloy. Due to its excellent high-temperature strength, superior radiation tolerance, and outstanding thermal and structural stability, the Cantor alloy has attracted attention as a potential structural material for next-generation nuclear power plants. Although previous studies have examined LSP effects on HEAs, most have focused on materials fabricated through laser-based additive manufacturing. However, such fabrication methods have limitations in terms of component size, reliability, and microstructural uniformity required for nuclear structural components, which are typically produced using conventional casting or forging processes. To address this issue, this study applies LSP to a cast CrMnFeCoNi alloy in order to better represent realistic manufacturing conditions. LSP was performed using different laser pulse energies, and the resulting changes in surface morphology, microstructure, and mechanical behavior were systematically characterized. The results reveal that increasing laser pulse energy leads to deeper plastic deformation while preserving the stable single-phase FCC structure without compositional variation. In addition, compressive residual stresses were successfully introduced on the treated surface. Pronounced microstructural evolution was observed, including significant grain refinement, an increased fraction of high-angle grain boundaries, elevated dislocation density, and the formation of sub-grain structures. These changes resulted in notable improvements in surface hardness and wear resistance. Overall, the findings demonstrate that LSP is a promising surface engineering approach for improving the performance of cast HEAs intended for nuclear structural applications.