Thiosalicylato complexes of Zn(II) and Hg(II), [Zn(SC₆H₄CO₂)(TMEDA)]₂ (1) and [TMEDA-H₂][Hg(SC₆H₄CO₂)₂] (2) where (TMEDA = N,N,N',N'-tetramethylethylenediamine) have been synthesized and characterized by spectral and single crystal X-ray analyses. Thiosalicylate opted different binding modes; in 1 all three binding sites are used whereas in 2 bonding of the ligands are mainly through S atoms, however, Hg-O distances indicate the existence of significant Hg^…O interactions. Bonding features have been explained on the basis of DFT calculations. Electronic absorption and emission spectral properties of both complexes have been studied. Electronic transitions have been calculated by TDDFT method and the results are consistent with the experimental spectra. We further have evaluated anti-bacterial, anti-biofilm and anti-tumour activities of 1 in addition to its cytotoxicity profile. It has been found to be highly effective against Escherichia coli, Klebsiella pneumoniae, Methicillin sensitive Staphylococcus aureus, Methicillin resistant Staphylococcus aureus (MRSA) and Pseudomonas aeruginosa clinical as well as reference strains. Interestingly, complex 1 has been found to be more potent than earlier reported Au(III) as well as Pt(II) thiosalicylate complexes. The effect of complex 1 on 72 h old biofilms of S. Aureus (MDR isolate 3) has been investigated by confocal laser scanning microscopy which revealed its bactericidal activities at the concentration as low as 0.125 µg/mL possibly by membrane alterations as evident by clear propidium iodide (PI) uptake. However, its antibiofilm activity is possibly due to interactions with biofilm matrix constituents.