A novel, simple and highly reliable method for the determination of trace amounts of mercury on a glassy carbon electrode (GCE) modified with a compound of crystal violet and multi-walled carbon nanotubes (CV+I−/MWCNTs) is reported. The principle of organic co-precipitation in solution was applied to the application of electrochemical analysis. First, the CV+/MWNTs modified electrode was immersed in solution with I−, CV+ and I− to form an ion association complex (CV+I−) as the carrier by the following reaction. Hg2+ was added into solution, where it reacted with I− to form HgI42−, then in the presence of CV+ an organic coprecipitator formed HgI42−·2CV+ by electrostatic adsorption. Finally, the HgI42−·2CV+ was adsorbed by the carrier CV+I− onto the surface of the modified electrode by the principle of organic co-precipitation. Meanwhile, trace mercury could be detected on the CV+I− and MWCNTs modified electrode (CV+I−/MWCNTs/GCE) by an electrochemical method. Therefore, preconcentration, separation and electrochemical detection of trace mercury were simultaneously and synchronously carried out. After optimization of the method, a linear response was obtained in the concentration range from 5.0 × 10−9 – 1.9 × 10−6 M, with a detection limit of 1.0 × 10−10 M. The modified electrode shows high selectivity, sensitivity and stability. This modified GCE does not present any significant interference from Cd2+, Cu2+, Ag+, Fe3+, Pb2+, Cr3+, Zn2+. No deterioration was observed in the electrode response during at least 3 weeks of successive measurements. The proposed method enables the separation, enrichment and electrochemical detection of trace mercury in aqueous solutions.