The new mercuric complex [Hg(HL)2Cl2] incorporating salicylaldimine ligand (HL = 2‐((pyridin‐3‐ylimino)methyl)phenol) was fabricated where the ligand molecules behaved in a monodentate manner via their pyridine nitrogen atoms. In addition to elemental characterization, X‐ray crystallographic studies of the complex revealed its packing in a monoclinic crystal system (space group: I2/a, a = 13.4276(2) Å, b = 6.20950(10) Å, c = 27.7530 (4) Å, α = γ = 90°, and β = 98.1610(10)°). Hydrothermal treatment of [Hg(HL)2Cl2] with thioacetamide afforded HgS microparticles (HgS μPs; Brunauer–Emmett–Teller [BET] surface area = 6.205 m2/g, diameter = 196.53–259.13 nm, and average size = 213.27 nm), whereas these microparticles were transformed to nanoscaled HgS particles (HgS NPs; BET surface area = 14.380 m2/g, diameter = 58.87–90.56 nm, and average size = 72.78 nm) by the action of ultrasonication. The as‐prepared HgS, HgS NPs in particular, afforded remarkable microbicidal activity against eight strains of filamentous and unicellular human pathogenic fungi and yeasts in comparison with cycloheximide. Remarkably, Aspergillus terreus grew up to 34.7 ± 1.88 mm in the absence of any fungicide, but HgS μPs, HgS NPs, and cycloheximide limited the fungal growth to 26 ± 0.94, 12.33 ± 1.6, and 28.3 ± 1.7 mm after incubation for 6 days. Besides, inhibition of Rhodotorula glutinis was of 7.6 ± 0.01 × 107 CFU/ml in control sample, but experiments included HgS μPs, HgS NPs, and cycloheximide limited the colony‐forming units of R. glutinis to 4.2 ± 0.01 × 107, 3.5 ± 0.02 × 107, and 5.9 ± 0.05 × 107 CFU/ml.