A single-phase FCC structure of the Fe₅₀Mn₂₀Ni₂₀Cr₁₀ high-entropy alloy (HEA) was fabricated by vacuum arc melting. The mechanical properties and deformation mechanisms at both ambient and cryogenic temperatures were systematically investigated. The results reveal that the Fe₅₀Mn₂₀Ni₂₀Cr₁₀ HEA exhibits an ultimate tensile strength of 763 ± 30 MPa and an elongation of 66 ± 3.2% at 77 K. These values represent 58% and 35% increases, respectively, when compared with the alloy’s tensile properties measured at ambient temperature. The significantly enhanced yield strength and sustained stable strain-hardening behavior owing to the deformation-induced twinning and phase-transformation capabilities at 77 K. The twinning-prone matrix is capable of inducing more pronounced twinning-induced plasticity (TWIP) effects, thereby contributing to superior ductility of the alloy. Owing to the significant enhancement in both strength and ductility at 77 K, this alloy provides a new perspective for the design of chemical composition and mechanical deformation behavior of high-performance HEAs used in low-temperature environments.