The strengthening due to the formation of strain-induced martensite in a low nickel medium manganese metastable austenitic stainless steel and the effect of different phases on subsequent aging was studied. Annealed 14wt.%Cr-8wt.%Mn-1wt.%Ni austenitic stainless steel was compressed uniaxially at 25 °C and 5 × 10–4 s−1 to different strains up to 0.8 true strain. The sigmoidal type of stress–strain curve obtained indicates strain-induced martensite formation during compression, with stress values higher than 2 GPa. Specimens compressed to a true strain of 0.41 and subsequently aged at 400 °C showed a peak hardness at 72 h. The hardness increased from about 520 HV1 after deformation to 575 HV1 at 400 ℃–72 h for 0.41 true strain. The microstructure was observed using an optical microscope and electron back-scattered diffraction technique. Ferritescope measurements showed that the fraction of strain-induced martensite increased with strain. The nano-hardness of individual phases i.e., austenite and martensite confirmed that the hardness of the martensite phase increased after aging and this increase was a function of the amount of strain imparted.