Herein the sulfur/nitrogen contained groups, serving as the "hooks" for electrochemical determination of Hg(II), were assembled on the reduced graphene oxide (hereafter SN-rGO) via a one-step facile hydrothermal method. The thiourea acts as a precursor for sulfur/nitrogen doping and partial reduction of graphene oxide. The SN-rGO was used to modify the glassy carbon electrode (GCE) for electrochemical detection of Hg(II) by square wave anodic stripping voltammetry (SWASV). The sulfur/nitrogen doping significantly improves the Hg(II) complexation by SN-rGO due to the creation of multifunctional groups on the graphene nanosheet. The SN-rGO modified electrode has excellent sensitivity (20.48 μA/μM) and limit of detection (LOD 8.93 nM) for Hg(II) detection. The newly developed Hg(II) sensing electrode possesses minimal interference for other ions typically found in natural waters. Therefore, it can be used for routine water quality monitoring. The fabrication of the SN-rGO electrode is rapid and low cost; hence, it offers a potential platform for environmental monitoring of toxic metal ions.