A fine-grained metastable dual-phase Fe₄₀Mn₂₀Co₂₀Cr₁₅Si₅ high entropy alloy (CS-HEA) with excellent strength and ductility was successfully prepared by friction stir processing (FSP). The microstructural and mechanical properties of the fine-grained CS-HEA were characterized. The results showed that as-cast shrinkage cavities and elemental segregation were eliminated. The average grain size was refined from 121.1 to 5.4 μm. The face-centered cubic phase fraction increased from 23% to 82%. During tensile deformation, dislocation slip dominated at strains ranging from 5% to 17%, followed by transformation induced plasticity (TRIP) from 17% to 26%, and twin induced plasticity (TWIP) from 26% to 37%. The yield strength, ultimate tensile strength, and elongation of the fine-grained CS-HEA were 503 MPa, 1120 MPa, and 37%, respectively. The strength−ductility synergy of fine-grained CS-HEA was attributed to the combined effects of TRIP, TWIP, dislocation strengthening, and fine-grained strengthening.