The reaction of 2-cyanopyridine or 2-cyanopyrazine with N-methylthiosemicarbazide afforded (Z)-2-(amino(pyridin-2-yl)methylene)-N-methylhydrazinecarbothioamide (HPyAm4M) and (Z)-2-(amino(pyrazin-2-yl)methylene)-N-methylhydrazinecarbothioamide (HPzAm4M), respectively. Prolonged heating under reflux in methanol in the synthesis of these thiosemicarbazones led to the formation of 4-methyl-5-pyridin-2-yl-2,4-dihydro-[1,2,4]triazole-5-thione (HMPytat, 1) and 4-methyl-5-pyrazin-2-yl-2,4-dihydro-[1,2,4]triazole-5-thione (HMPztat, 2) by oxidative cyclization of the starting thiosemicarbazones. Reactions of thiosemicarbazones with mercury(II) salts gave new complexes of general formulae [Hg(tat)X] (3, 4) and [Hg(Htat)₂X₂] (5, 7, 9–11), along with [Hg₃(HEPytat)₂(EPytat)₂Cl₄]·3H₂O (6) and [Hg(EPytat)(HEPytat)I]·H₂O (8) (Htat = general 1,2,4,-triazole-5-thione). The complexes were isolated as solids and were characterized by elemental analysis, mass spectrometry and IR, ¹H NMR and ¹³C NMR spectroscopy. Recrystallization of the complexes from DMSO gave the compounds [Hg(MPytat)₂]_n (3a), [Hg(HMPytat)₂Br₂]·H₂O (4a), [{Hg(HEPytat)(μ²-NS-EPytat)Cl₂}₂Hg] (6a), [Hg(EPytat)₂]_n (6b), [Hg(MPztat)₂]_n (10a) and [Hg(EPztat)₂]_n (11a) (HEPytat = 4-ethyl-5-pyridin-2-yl-2,4-dihydro-[1,2,4]triazole-5-thione and HEPztat = 4-ethyl-5-pyrazin-2-yl-2,4-dihydro-[1,2,4]triazole-5-thione). These complexes were studied by single-crystal X-ray diffraction. The structural analysis revealed the versatility of heterocyclic functionalized 1,2,4-triazole-5-thiones to coordinate to mercury(II) in the thione and thiolate forms through the use of various donor atoms to give different and interesting crystalline packings. The supramolecular assemblies observed in the solid state were analysed. These elegant assemblies are formed by a combination of several noncovalent interactions, including intermolecular hydrogen-bonding supramolecular synthons and aromatic–aromatic interactions. The short Hg⋯Hg distance of 3.660 Å indicates the presence of a probable mercurophilic interaction in 6a.