In this study, we have produced strontium hexaferrite (SrFe12O19) magnets by mechanochemically synthesizing the mill scale which is the waste material from the hot rolling process of steel slabs and strontium carbonate (SrCO3) powders. The mechanochemical synthesis process was conducted via high energy ball milling process. The stoichiometric ratio (SrCO3/Fe2O3) was changed from 1:5.5 to 1:6.6 by 0.1 increments, and the influence of stoichiometric ratio was investigated with regards to phase structure and magnetic properties. The relationship between the magnetic performances and structures was well established through Vibrating Sample Magnetometry (VSM) measurements and Rietveld refinement analysis of powder X-ray diffraction data. The primary phase formed in powder structures for all the stoichiometric ratios was SrFe12O19 and the other phases of alpha-Fe2O3, SrO, and SrFe2O4 with varying amount, depending on the stoichiometric ratios, were also obtained. The maximum magnetic properties were obtained with 1:6.0 SrCO3/Fe2O3 stoichiometric ratio. The coercivity (H-c), the saturated magnetic flux density (B-s), the residual magnetic flux density (B-r), and maximum energy product (BH)(max) values for 1:6.0 stoichiometric ratio were obtained as 3682 Oe, 506 mT, 311 mT, and 3.11 MGOe, respectively.