Chemically modified electrode (CME) has shown significant potential for achieving on-site and real-time detection of heavy metal ions. In this paper, a nanocrystalline SnO2-modified Au electrode as an electrochemical sensor for ultrasensitive mercury ion (Hg2+) detection was proposed and demonstrated. The modification method is simply employing the spin-coating process of colloidal SnO2 quantum wires at room temperature in ambient air without further heat treatment. Nanocrystalline SnO2 possesses a large surface area with abundant dangling bonds, through which the enrichment and dissolution of ions were enhanced and converted to significant current signal under anodic stripping voltammetry (ASV) scanning mode. According to the results of DFT analysis, SnO2 has a greater adsorption energy for Hg2+ than that of Au, which further improves its sensitivity. The nanocrystalline SnO2-modified electrode exhibits sensitive response to Hg2+ in the range of 10−9 M~10−4 M, with the limit-of-detection (LOD) of 0.036 nM. The selectivity was ascribed to the higher adsorption energy of Hg on SnO2 surface compared with other metals in this study. The electrode modification strategy based on colloidal semiconductor nanocrystals may promote the design of sensitive sensors for rapid and reliable ion detection.