•Three 1-picolinoyl-4-substituted thiosemicarbazides and six corresponding heteroleptic Pd(II) and Pt(II) complexes have been synthesized, structurally determined and theoretically studied.•Antimicrobial activities against Gram-positive bacteria, Gram-negative bacteria and yeast as well as anticancer activities against MCF7 and HepG2 cancer cells of the compounds were investigated.•The in vitro cytotoxicity against human cancerous cell lines of most metal complexes is significantly higher than that of corresponding organic ligands and comparable with cisplatin under the same condition. Reactions of three 1-picolinoyl-4-substituted thiosemicarbazides H2L, namely H2LEt, H2LCy, and H2LPh, with equimolar amounts of common Pd(II) or Pt(II) complexes M(PPh3)2Cl2 (M = Pd, Pt) bring about the formation of stable heteroleptic complexes with the compositions M(L)(PPh3) (M = Pd, Pt). Their molecular structures show mononuclear square planar complexes in which the acyl thiosemicarbazides are doubly deprotonated and serve as tridentate ligands bonded to the metal ion center via (Npyridine, N1, S) donor sets. The organic ligands H2LEt and H2LPh as well as all of their metal complexes show only weak inhibitory effects on microbial strains examined, whereas the metal complexes of H2LCy exhibit good inhibition towards B. subtilis and L. fermentum with MIC values lower than those of ampicillin. Furthermore, the in vitro cytotoxicity of H2L and the complexes were tested against cancerous cell lines MCF7 and HepG2. The organic ligands exhibit moderate antiproliferative effects with the increasing order of activity H2LEt < H2LCy < H2LPh for both cell lines. Surprisingly, except for the Pt(II) complex of H2LCy, the other metal complexes are highly toxic with IC50 in the range of 4.22±0.37 – 16.0±1.05 μM for HepG2 and 3.26±0.41 – 20.6±2.12 μM for MCF7. Most IC50 values of the metal-based compounds are comparable with those of cisplatin under the same condition. Display omitted