We report an extremely sensitive and specific detection of mercuric ions (Hg2+) based on graphene assisted laser desorption/ionization mass spectrometry (GALDI-MS). Combining the highly selective coordination interactions between thymine (T) and Hg2+, we present a simple, effective, and novel approach, based on π–π interactions of the T-Hg2+-T complex and G that can serve as a platform and matrix for GALDI-MS. The present sensor not only exhibits high selectivity and sensitivity (picomolar) to Hg2+ in aqueous solution, but also can elucidate the chemical structures of the metal complexes. The significant advantage in the current approach is that there is no need for a sophisticated instrument, and no sample pretreatment is required to detect the Hg2+ ions.