The study investigates the exceptional damage-tolerance properties of a medium-entropy alloy (MEA) CrCoNi at cryogenic temperatures. The CrCoNi alloy, an equiatomic face-centred cubic (fcc) solid solution, exhibits superior strength, ductility, and fracture toughness compared to both high-entropy alloys (HEAs) and multi-phase alloys. At room temperature, the alloy shows tensile strengths of nearly 1 GPa, failure strains of ~70%, and fracture toughness values above 200 MPa m^1/2. At cryogenic temperatures, these properties improve further, with strength levels above 1.3 GPa, failure strains up to 90%, and fracture toughness values of 275 MPa m^1/2. The enhanced performance is attributed to continuous steady strain hardening, which suppresses plastic instability through pronounced dislocation activity and deformation-induced nano-twinning. The CrCoNi alloy's unique combination of strength, ductility, and toughness makes it one of the toughest metallic materials reported to date, with potential applications in structural materials requiring high damage tolerance.The study investigates the exceptional damage-tolerance properties of a medium-entropy alloy (MEA) CrCoNi at cryogenic temperatures. The CrCoNi alloy, an equiatomic face-centred cubic (fcc) solid solution, exhibits superior strength, ductility, and fracture toughness compared to both high-entropy alloys (HEAs) and multi-phase alloys. At room temperature, the alloy shows tensile strengths of nearly 1 GPa, failure strains of ~70%, and fracture toughness values above 200 MPa m^1/2. At cryogenic temperatures, these properties improve further, with strength levels above 1.3 GPa, failure strains up to 90%, and fracture toughness values of 275 MPa m^1/2. The enhanced performance is attributed to continuous steady strain hardening, which suppresses plastic instability through pronounced dislocation activity and deformation-induced nano-twinning. The CrCoNi alloy's unique combination of strength, ductility, and toughness makes it one of the toughest metallic materials reported to date, with potential applications in structural materials requiring high damage tolerance.