HSLA steel is welded to AA 5083 by FSW technique in butt configuration and investigated for localized mechanical properties across the sheet thickness in the proximity of the interface. The quality of the weld joint was examined by macro-scale analysis, while APT data confirmed the atomic mixing status of the joint interface. The localized mechanical properties were assessed by a nanoindentation study along the interface. The variation in hardness and Young's modulus along the sheet thickness is reported. The hardness of the interface from crown to crest across thickness of the weld joint varies from ~2.7 to 5.8 GPa while Young's modulus of the region fluctuates between ~178 to 222 GPa and revealed a clear gradient over sheet thickness. The corresponding interface structure was examined by XRD2, EBSD and TEM study. The observed results collectively exhibited the distinct segregation of Fe4Al13, Fe2Al9, Fe2Al5 and Fe3Al IMC layers over the weld crown to crest (sheet thickness) with different combinations of stoichiometric variety and their quantitative presence which seems to be accountable for the noted property gradient. Overall, the nanoindentation data along with the associated microstructure analysis results of the FSW interface for the stated pair of alloys are discussed in the paper in the context of deleterious IMC layers and localized mechanical properties along the weld crown to crest.