A new series of Ni (II), Zn (II), and Cu (II) mixed-ligand complexes of pentamethylenedithiocarbamate (pmdtc) with diamines such as ethylenediamine (en), diethylenetriamine (dien), and triethylenetetramine (trien) are reported. The synthesized complexes were characterized by elemental, thermal, and magnetic analyses, as well as UV–Visible, infrared (IR), Nuclear Magnetic Resonance (NMR), and Electron Spin Resonance (ESR) spectral studies. The antibacterial, antifungal, and anticancer activities of these complexes were also investigated. All the complexes exhibited reasonable biological activity, indicating a promising future for the mixed-ligand complexes of Ni (II), Zn (II), and Cu (II) in the biological field with potentially fewer side effects.