This study demonstrates that the SMAT (surface mechanical attrition treatment, which is an SSD process) changes the electrochemical and plasma-nitriding (at 520 °C for 10 h) response of the austenitic stainless steel (AISI 304). Higher ball velocity in the SMAT process forms denser deformation twins/shear bands. The SMAT process causes deformation-induced martensitic (DIM) transformation and grain refinement. A higher ball velocity in the SMAT results in the transformation of a higher austenite volume into DIM. The maximum hardness is observed at the SMATed surface. The depth of the hardened layer and surface hardness increase with ball velocity. The SMAT process enhances the passive film’s stability, resulting in a lower corrosion rate and higher pitting potential. SMAT reduces the effective nitriding time, causing a thinner nitrided layer. High nitrogen (linked to the expanded austenite phase) and the formation of other phases like ferrite, martensite, and CrN, significantly enhance the nitrided sample’s hardness to ~1200 HV0.1. The SMAT and nitriding processes have improved the material’s wear resistance by ~26 and ~77%, respectively. SMATed sample displays the lowest COF among the studied samples.