High manganese lightweight steel alloys in the Fe-Mn-Al-C system are promising alternatives to quenched and tempered Cr and Mo steels for transportation and military applications. The understanding of nonmetallic inclusion formation and their effect on the mechanical properties is of extreme importance for further alloy development. Sharp and brittle AlN forms prior to the liquidus and has been shown to decrease notch toughness. Controlled additions of sulfur may promote soft and globular MnS that precipitates around AlN during solidification, thus mitigating their detrimental effect. The effect of controlled sulfur additions from 0.004 to 0.042%S was studied in a Fe-30%Mn-9%Al-1%Si-(0.9-1.2)%C-0.5%Mo steel. The main inclusions observed were AlN, MnS, and AlN-cored MnS. Charpy impact toughness was evaluated in the solution treated condition and in specimens aged to 329-340 HBN. Toughness is a function of the overall inclusion population. In aged specimens, same was true only for steels with 0.9% carbon; above 1%C the toughness was below 15 J for any sulfur content.