A novel tetradentate azo‐Schiff base ligand (H2L) was synthesized by 2:1 molar condensation of an azo‐aldehyde and ethylenediamine. Its mononuclear Cu(II), Ni(II), Co(II) and Zn(II) complexes were prepared and their structures were confirmed using elemental analysis, NMR, infrared and UV–visible spectroscopies and molar conductivity measurements. The results suggest that the metal ion is bonded to the tetradentate ligand through phenolic oxygens and imine nitrogens of the ligand. The solid‐state structures of the azo‐Schiff base ligand and its Cu(II) complex were determined using single‐crystal X‐ray diffraction studies. The azo‐Schiff base ligand lies on a crystallographic inversion centre and thus the asymmetric unit contains half of the molecule. X‐ray data revealed that keto–amine tautomer is favoured in the solid‐state structure of the ligand. In the structure of the Cu(II) complex, the Cu(II) ion is coordinated to two phenolate oxygen atoms and two imine nitrogen atoms of the azo‐Schiff base ligand with approximate square planar geometry. The anticancer activity of the synthesized complexes was investigated for human cancer cell line (MCF‐7) and cytotoxicity of the synthesized compounds was determined against mouse fibroblast cells (L929). The ligand and its complexes were found to show antitumor activity. The synthesized metal complexes were optimized at the B3LYP/LANL2DZ level and a new theoretical formula for MCF‐7 cells was also derived.