The manuscript presents a systematic study for the characterization of mild steel under varying stress-triaxiality, strain rate and temperature. The effect of stress triaxiality was studied by performing tension tests on notched cylindrical specimens. The radius of the notch has been varied as 0.2, 0.4, 0.6, 0.8, 1, 2, 3, 4, 5 and 10 mm. The smooth cylindrical specimens were tested under tension at varying strain rate and temperature. The strain rate in the range, 0.0006 s⁻¹–1500 s⁻¹, was obtained on a universal testing machine and the Hopkinson pressure bar apparatus. The tests at elevated temperature were carried out using a portable furnace which enabled the variation of temperature from 100 °C to 750 °C. All the material parameters for the Johnson–Cook elasto-viscoplastic material model have been calibrated. The parameters thus obtained were validated by numerically simulating the material characterization under high strain rate using ABAQUS/Explicit finite element code. The numerical simulations were also carried out for the ballistic evaluation of 12 and 16 mm thick mild steel plates impacted by 7.62 AP projectiles at varying incidence velocities in order to obtain the ballistic limit. The angle of incidence has also been varied until the occurrence of critical ricochet. The numerical results have been validated by the experiments reported in earlier studies, Gupta and Madhu [1,2].