A systematic study was carried out to understand the effect of Nb addition on the microstructure, phase stability, and mechanical properties in a series of cast and aged CoCrFeNi2.1Nbx (x = 0, 0.2, 0.4, 0.74) high entropy alloys (HEAs). The base alloy without Nb addition showed a fully face-centered cubic (FCC) phase, whereas Nb addition was found to promote the formation of the hexagonal Laves phase. The cast HEAs were further aged at temperatures varying from 800 °C to 1200 °C. Aging of the different Nb alloyed HEAs at 800 °C and 1000 °C resulted in the precipitation of D019 structured ε-phase having fine acicular morphology. In contrast, the HEAs aged at 1200 °C did not reveal the presence of ε precipitates. These experimental observations were critically assessed with the thermodynamic predictions. The fine precipitate network in the CoCrFeNi2.1Nb0.2 and CoCrFeNi2.1Nb0.4 HEAs improved the hardness and tensile strength considerably. However, the CoCrFeNi2.1Nb0.74 HEA showed considerable embrittlement due to the presence of a high fraction of the Laves phase. Present results indicated considerable potential of achieving superior mechanical properties in these cast HEAs even without involving thermo-mechanical processing. © 2020 Elsevier B.V.