Activated carbon (AC) was modified with urea, thioglycolic acid and thiourea to obtain nitrogen doped activated carbon (ACN), sulfur doped activated carbon (ACS) and nitrogen and sulfur co-doped activated carbon (ACNS), respectively. The AC samples were characterized by elemental analysis, N2 adsorption-desorption, determination of the pH of the point of zero charge (pHpzc) and X-ray photoelectron spectroscopy, and tested for adsorption behaviors of Hg(II) ions. The experimental data of equilibrium isotherms fitted well with the Langmuir model. ACNS showed the highest adsorption capacity of 511.78 mg/g, increasing more than 2.5 times compared to the original ACA. The adsorption process followed pseudo-second-order kinetics. The thermodynamic parameters of ΔH°, ΔS°, and ΔG° at 30 °C were −20.57 kJ/mol, −0.032 kJ/mol K and −10.87 kJ/mol, respectively. It was concluded that the Hg(II) ions' adsorption on ACNS was exothermic, spontaneous and physiosorptive in nature. Finally, the adsorption capacity of ACNS reduced by just 8.13% even after the sixth cycle compared to the initial cycle.