In this study, a quinoline–thiourea conjugate (1-phenyl-3-(quinoline-8-yl) thiourea, PQT) was synthesized and used as a fluorescence sensor to detect mercury ion. The observation is coincident with the well-documented phenomenon that a thiocarbonyl-containing group on a fluorochrome quenches the fluorescence due to the heavy atom effect of the S atom. The large fluorescence enhancement of PQT in the buffered MeCN–water mixture (1/1 v/v; HEPES 100 mM; pH 8.0) was caused by the Hg²⁺ induced transformation of the thiourea function into a urea group. As such, protic solvents can be ascribed to hydrogen bond formation on the carbonyl oxygen to reduce the internal conversion rate. The fluorescence intensity of PQT was enhanced quantitatively with an increase in the concentration of mercury ion. The limit of detection of Hg²⁺ was 7.5 nM. The coexistence of other metal ions with mercury had no obvious influence on the detection of mercury. A quinolone–thiourea conjugate was used as a fluorescent probe to detect Hg²⁺ in aquatic plants and the experimental results were satisfactory.