||We developed a liquid crystal (LC) sensor system for detecting mercuric ion (Hg2+) in aqueous solutions. In this system, 4-cyano-4′-pentyl biphenyl (5CB) was doped with a sulfur- and nitrogen-containing ligand 5-(pyridine-4-yl)-2-(5-(pyridin-4-yl)thiophen-2-yl)thiazole (ZT) as the Hg2+ specific LCs. When the system was immersed in the solution containing Hg2+, the complex of ZT and Hg2+ formed, which disrupted the orientation of LC and lead to a dark-to-bright transition of the image of LCs. From mercuric binding titrations monitored by UV–vis spectroscopy, it was found that 1:1 Hg2+/ZT complexes were formed. The limit of detection (LOD) of the system to Hg2+ is 10 μM, and it did not respond to Cd2+, Zn2+, Cu2+, Pb2+, Fe+, Mg2+, Ca2+, Na+, and K+. Besides, we also demonstrated that this system is capable of detecting Hg2+ in tap water and pond water. Because the signal of this system is colorful under ambient light, which is readily understood by normal users, it can be used as a portable device to monitor the water quality at any location.