We have synthesized two isostructural mononuclear cobalt(III) complexes [1]NO₃·3H₂O and [1]NO₃·CH₃CO₂H·H₂O {[1]₊ = [Co(1,10-phenanthroline)₂Cl₂]⁺} and characterized by single crystal X-ray structural analyses. Mass spectral studies of the complexes indicate both the compounds to produce identical cationic species viz., [Co(phen)₂Cl₂]⁺ in methanol solution. [1]⁺ has been evaluated as model system for the catechol oxidase enzyme by using 3,5-di-tert-butylcatechol (3,5-DTBC) as the substrate in methanol medium, which revealed that the cationic complex efficiently inhibits catalytic activity with k_cat value 9.65 × 10² h⁻¹. [1]⁺ cleaved pBR 322 DNA without addition of an activating agent. Further, the anti-cancer activity of [1]⁺ on human hepatocarcinoma cell line (HepG2) has been examined. The induction of apoptosis induced in the cell line was assessed base on the changes in cell morphology, which showed the efficacy of [1]⁺ to induce apoptosis in 53% of cells during 24 h treatment. Interestingly, the observed IC₅₀ values reveal that [1]⁺ brings about conformational change on DNA strongly and exhibits remarkable cytotoxicity.