At low temperatures most metals show reduced ductility and impact toughness. Here, we report a compositionally lean, fine-grained Fe-30Mn-0.11C austenitic steel that breaks this rule, exhibiting an increase in strength, elongation and Charpy impact toughness with decreasing temperature. A Charpy impact energy of 453 J is achieved at liquid nitrogen temperatures, which is about four to five times that of conventional cryogenic austenitic steels. The high toughness is attributed to manganese and carbon austenite stabilizing elements, coupled with a reduction in grain size to the near-micrometer scale. Under these conditions dislocation slip and deformation twinning are the main deformation mechanisms, while embrittlement by α′- and ε-martensite transformations are inhibited. This reduces local stress and strain concentration, thereby retarding crack nucleation and prolonging work-hardening. The alloy is low-cost and can be processed by conventional production processes, making it suitable for low-temperature applications in industry.