Cobalt(III) complexes [Co(pnt)(B)₂](NO₃)₂ (1–3) of pyridine-2-thiol (pnt) and phenanthroline bases (B), viz. 1,10-phenanthroline (phen in 1), dipyrido[3,2-d:2′,3′-f]quinoxaline (dpq in 2) and dipyrido[3,2-a:2′,3′-c]phenazine (dppz in 3), have been prepared, characterized and their photo-induced anaerobic DNA cleavage activity studied. The crystal structure of 1a as mixed ClO₄⁻ and PF₆⁻ salt of 1 shows a Co^IIIN₅S coordination geometry in which the pnt and phen showed N,S- and N,N-donor binding modes, respectively. The complexes exhibit Co(III)/Co(II) redox couple near −0.3 V (vs. SCE) in 20% DMF–Tris-HCl buffer having 0.1 M TBAP. The complexes show binding propensity to calf thymus DNA giving K_b values within 2.2 × 10⁴–7.3 × 10⁵ M⁻¹. Thermal melting and viscosity data suggest DNA surface and/or groove binding of the complexes. The complexes show significant anaerobic DNA cleavage activity in red light under argon atmosphere possibly involving sulfide anion radical or thiyl radical species. The DNA cleavage reaction under aerobic medium in red light is found to involve both singlet oxygen and hydroxyl radical pathways. The dppz complex 3 shows non-specific BSA and lysozyme protein cleavage activity in UV-A light of 365 nm via both hydroxyl and singlet oxygen pathways. The dppz complex 3 exhibits photocytotoxicity in HeLa cervical cancer cells giving IC₅₀ values of 767 nM and 19.38 μM in UV-A light of 365 nm and in the dark, respectively. A significant reduction of the dark toxicity of the dppz base (IC₅₀ = 8.34 μM in dark) is observed on binding to the cobalt(III) center.