The syntheses of a new Schiff base, diethyl-5-(2-hydroxybenzylidene)aminoisophthalate (HL), and a copper complex, [Cu(L-2)] (1), imparting L, have been described. Both the ligand HL and complex 1 have been thoroughly characterized by elemental analyses, electrospray ionization mass spectrometry, FT-IR, NMR (H-1 and C-13), electronic absorption, and emission spectral studies and their structures determined by X-ray single-crystal analyses. Distinctive chemodosimetric behavior of HL and 1 toward Hg2+ has been established by UV/vis, emission, and mass spectral studies. Comparative studies further revealed that the chemodosimetric response solely originates from selective hydrolysis of the aldimine moiety over the ester group and 1 exhibited greater selectivity toward Hg2+ relative to HL while the sensitivity order is reversed. Further, these followed different hydrolytic pathways but ended up with the same product analyzed for diethyl-5-aminoisophthalate (DEA). Hg2+-induced displacement of Cu2+ and subsequent hydrolysis of the -HC=N-moiety in 1 affirmed the identity of the actual species undergoing hydrolysis as HL. The occurrence of Cu2+ displacement and Hg2+ detection via hydrolytic transformation has been supported by various physicochemical studies.