Possibilities of tailoring microstructure and mechanical properties of complex intermetallic containing CoCrFeNi2.1Nbx (x = 0.2, 0.4) high entropy alloys (HEAs) using thermo-mechanical processing were investigated in this work. For this purpose, the homogenized HEAs containing complex Laves phases distributed in the FCC matrix were successfully severely cold-rolled up to 90% reduction in thickness. The cold-rolled HEAs showed a nano lamellar microstructure with fragmented Laves phase particles arranged preferentially along the rolling direction. The microstructure of the Nb0.4 HEA revealed intense deformation zones surrounding the hard Laves phase particles, owing to their higher volume fraction and larger size as compared to those in the Nb0.2 HEA. Upon annealing at 800 °C, the lamellar deformed FCC phase in the two HEAs was transformed into an equiaxed nano-grained structure having fine dispersion of nano precipitates. However, the Nb0.4 showed a decidedly lower recrystallized grain size than the Nb0.2, resulting from the smaller deformed grain size and particle stimulated nucleation. The annealed Nb0.2 showed remarkable strength-ductility combination featured by tensile strength ~1080 MPa and ductility ~21%, originating from the combined effect of a high volume of ductile FCC matrix, remarkable grain refinement, and precipitation hardening. In stark contrast, the annealed Nb0.4 showed complete absence of tensile elongation, likely due to a low volume fraction of the ductile FCC matrix and a high volume fraction of the intermetallic phases. The present study emphasized the critical role of microstructural tailoring for achieving outstanding strength-ductility synergy in complex intermetallic containing CoCrFeNi2.1Nbx HEAs. © 2019 Elsevier B.V.