In the present work, multi walled carbon nanotubes (MWCNT) reinforced magnesium (Mg) matrix composite was fabricated by friction stir processing (FSP) with an aim to explore its mechanical and electrochemical behavior. Microstructural observations showed that the thickness of the produced composite layer was in the range of 2500 μm. FSP resulted uniform distribution of CNT near the surface while agglomerated layers in the subsurface. Grain refinement of Mg achieved by FSP improved the hardness but significant enhancement in the hardness value was observed for FSPed MWCNT/Mg composites. Potentiodynamic polarization studies revealed that the increase in corrosion current density was observed for MWCNT/Mg composite compared with grain refined Mg and pure Mg, implying the significance of secondary phase (MWCNT) in decreasing the corrosion resistance of the composite.