New iron(III) complexes of 5-methyl-3-formylpyrazole-N(4)-dimethylthiosemicarbazone (HMPZNMe2) (I) and 5-methyl-3-formylpyrazole-N(4)-diethylthiosemicarbazone (HMPZNEt2) (III), [Fe(MPZNMe2)2]NO3·H2O (II) and [Fe(MPZNEt2)2]NO3·H2O (IV), respectively, have been synthesised for the first time and physico-chemically characterised by magnetic measurements (polycrystalline state), electronic and IR spectra. Both are cationic complexes containing two monodeprotonated tridentate ligands with NNS donor sites and an anionic counterpart; the complex species behave as 1:1 electrolytes in MeOH. IR spectra (4000–200 cm−1) indicate coordination to iron(III) centre via the pyrazolyl (tertiary) ring nitrogen, azomethine nitrogen and thiolato sulfur atom. X-ray crystallographic data of (II) (monoclinic, C2/c) and (IV) (P1¯, triclinic) have authenticated the FeN4S2 octahedral coordination in both complex ions, as envisaged from the spectral data. In both species, the two azomethine nitrogen atoms are trans to each other, while the pyrazolyl ring nitrogen and the thiolato sulfurs atoms are in cis positions. We further observed that both the ligands and their corresponding complexes were capable of killing HeLa cells in culture, but the cytotoxicity of the complexes was found to be greater than their corresponding ligands. Complex (II) does not have any role in cell-cycle progression, but complex (IV) arrests the M-phase to prevent cell-cycle progression.