New copper complexes of 2-hydroxy-5-methoxyacetophenone thiosemicarbazone and its N(4)- substituted derivatives were synthesized and characterized by theoretical DFT studies and experimental UV–Vis, FT-IR, EPR spectral analysis, cyclic voltammetry, magnetic susceptibility and conductivity measurements. The DFT calculation results have been used to predict and interpret the experimental results. The geometric parameter G within the range of 7.61–7.86 for all complexes confirms the mononuclear nature of the complexes. The EPR, UV-Vis, DFT studies and obtained bonding parameters show that all the complexes have square planar geometry and their M-L bonds have strong ionic and some in-plane σ-bond character. In addition, the experimental and DFT studies showed that HOMO and LUMO energy levels of the complexes may present good electron transporting properties. Also, the investigated Cu(II) complexes were tested for biological activity, proving both in vitro antibacterial and anticancer activity. The complexes exhibited antibacterial activity against Gram positive bacteria S. aureus while exhibiting no activities against gram negative bacteria E. coli and S. gallinarum. The f parameters obtained experimentally by EPR support the antimicrobial activity properties results of the complexes. The evaluations of potential anticancer activity of these complexes were carried out against highly metastatic MDA-MB-231 breast adenocarcinoma cell line by MTT assay. Our results suggest that all tested copper complexes have high cytotoxic effects with the range of 1.76–3.53 μM IC50values in vitro. These copper complexes could be considered as potential anticancer agents to counteract drug resistance of metastatic cancer cells.