The influences of sulfur, HNO3, and thermal treatments on the properties and Hg adsorption of activated carbon (AC) derived from bituminous coals were evaluated. These ACs were impregnated with either polysulfide or elemental sulfur at 120, 200, and 600°C. Additional ACs were treated with HNO3 at 95°C to incorporate oxygen functionalities, and then subjected to temperature-programmed desorption to 950°C to remove surface oxygen groups and create nascent active sites on the AC surface. Polysulfide and elemental sulfur impregnation at < 200°C did not improve adsorption. However, improvement in HgCl2 adsorption by up to 97% was observed. Elemental sulfur impregnation at 600°C enhanced Hg0 and HgCl2 adsorption by up to 42% and 404%, respectively, for ACs derived from a low-organic-sulfur coal. Improvements in Hg0 adsorption for ACs from high-organic-sulfur coals were not observed after sulfur impregnation. HNO3 and thermal treatments reduced Hg-active groups such as sulfur. Oxygen surface groups and the nascent carbon sites appeared to be more reactive towards other flue gas components than Hg, because HNO3 and thermal treatments led to a reduction in Hg0 and HgCl2 adsorption. These results suggest that AC’s physical and chemical properties after sulfur treatment influence Hg0 adsorption, while HgCl2 adsorption is mainly affected by AC’s chemical characteristics.