The effect of C and Mo alloying elements on the microstructure and mechanical properties of transformation-induced plasticity (TRIP) high-entropy alloys (HEAs, Fe50Mn30Co10Cr10) was systematically investigated. The results showed that the as-cast TRIP-HEA doped with 2 at.% C presents an excellent combination of high tensile strength (similar to 600 MPa) and high ductility (similar to 67.4%) owing to solid-solution strengthening. Surprisingly, the addition of 1 at.% Mo promotes the improvement in tensile strength (similar to 658 MPa) and significant increase in elongation (similar to 89.8%), which is much larger than that of the most reported as-cast HEAs. This is attributed to the formation of finer equiaxed dendrite grain and the twinning-induced plasticity effect with more deformation twins in TRIP-HEAs added by C and Mo elements.