The flow stress of Hadfield steel is attributed to lattice friction, dynamic strain aging (DSA), mechanical twinning, and forest hardening. However, it has not been clarified yet which mechanism quantitatively contributes most to the work hardening. Here, an austenitic Hadfield steel containing 1.1 wt.% C and 12.5 wt.% Mn was studied, and tensile tests at a strain rate of 1 s−1 were performed to neglect the DSA effect. The lattice friction was estimated by the Hall–Petch relationship. Twin plates characteristics and dislocation densities were estimated quantitatively by TEM and XRD, at 11, 18, 25, 38, and 55% strain levels. A linear approach was utilized to model the flow stress. At 11 and 18% strain, the role of mechanical twinning dominated the work hardening by contributions to the flow stress with 195 and 261.6 MPa higher than the forest hardening contributions which recorded 135.5 and 250.2 MPa, respectively. With the strain level increase, the thicknesses of the twinned plates were increased from 30 to 125 nm at 11 and 55% strain, respectively, resulting in a reduced contribution by the mechanical twinning while a rising effect of dislocations was noticed at large strain levels (38 and 55%) recording contributions of forest hardening to the flow stress with 437.2 and 544 MPa higher than the mechanical twinning contributions which likewise recorded 416.7 and 500.5 MPa, respectively.