1,2,4-Triazine derivatives showed a significant role in the biological and pharmacological fields, which motivated us to study the biological and cytotoxicity activity of its first-row transition metal complexes. Three new complexes of Co(II), 1; Ni(II), 2; and Cu(II), 3 with 3-(2-benzylidenehydrazinyl)-5-(4-isobutylphenyl)-1,2,4-triazine, L were synthesized and characterized by standard physicochemical and theoretical methods. The structure of these complexes was evidenced by IR and 1H NMR spectroscopies, elemental analysis, magnetic measurements, molar conductance, mass spectrometry, and DFT modeling. The ligand behaved as a bidentate chelator that coordinates by triazine nitrogen-2 and azomethine nitrogen in a 2:1 (ligand : metal) ratio. The anticancer activity of the compounds was investigated against breast cancer cells (Mcf7 cell line). The antimicrobial activities of the ligand and its complexes were evaluated against different types of bacteria. Cu(II) was the most efficacious antimicrobial, with greater inhibition zones than the other complexes. The molecular docking studies were implemented to consider the binding attraction of the compounds with the receptors of breast cancer oxidoreductase (PDB:3HB5) and those of the COVID-19 main protease viral protein (PDB ID: 6LU7). The DFT calculations were used to confirm the molecular geometry of the new complexes.