Pb (II) and Hg (II) ions were reacted in aqueous solution with two tridentate thiosemicarbazones, resulting in four complexes {[Pb(L¹)₂] 1, [Pb(L²)₂] 2, [Hg(L¹)₂] 3 and [Hg(L²)₂] 4; HL¹ = 4-(4-nitrophenyl)-1-((pyridin-2-yl)methylene)thiosemicarbazide and HL² = 4-(2-chlorophenyl)-1-((pyridin-2-yl)methylene)thiosemicarbazide}. The solid-state structures of complexes 2 and 3 were determined by X-ray crystallography, which also confirmed the monobasic tridentate NNS nature of the ligands, and the octahedral coordination geometry around the metal centers. Complexes 1-4 exhibited high antifungal activity against the pathogen Cladosporium sphaerospermum; all four complexes giving rise to a reduction in the fungal dry mass and sugar consumption. Complexes 2 and 4 that inhibited Cladosporium sphaerospermum growth at the concentration of 1.8 mM are more efficient inhibitors than the free ligand and metal ions. Further, the complexes gave varied values of the fungal total antioxidants (TA), superoxide dismutase (SOD) and catalase (CAT) activities dependent on their concentrations.