In this study, the role of mechanochemical pre-treatment with the addition of S in atmospheric agitation leaching of a lateritic nickel ore was investigated. In the mechanically activated sample with S (20 wt%), the amount of Ni-Co-bearing amorphous Fe Mg hydrosilicate phases decreased and a new FeMgS phase was formed. In addition, the formation of NiSO4 was determined by X-ray photoelectron spectroscopy analysis. The effect of leaching temperature, leaching time, H2SO4 concentration, and the amount of S added during the mechanochemical pre-treatment on the extractability of Ni, Fe, and Co were investigated. When the mechanically activated sample with 20% added S was leached for 2 h under conditions of a solid/liquid ratio of 1/2.5 (w/w), temperature of 85 degrees C, and H2SO4 concentration of 300 g/L, 92.0% Ni, 79.1% Fe, and 84.8% Co extractions were achieved. Moreover, 17.5% lower Ni extraction was obtained using the mechanically activated sample without S. In terms of leaching behavior, the relationship between Ni and Fe dissolution weakened after mechanochemical activation. Furthermore, the leaching of the newly formed compounds was mostly controlled by the chemical reaction model.