The present study delineates the biosynthesis of ZnOSA NPs by availing S. androgynous (L.) aqueous leaf extract. The formation of ZnOSA NPs was substantiated by FT-IR and UV–visible spectroscopy which was further corroborated by the elemental composition study through EDS spectroscopy. The FE-SEM and HRTEM micrographs have revealed the formation of NPs with different shapes and sizes ranging from 12 to 23 nm. The BET isotherms obtained have manifested the mesoporous nature of ZnOSA NPs. The in-vitro cytotoxicity studies of ZnOSA NPs carried out on MDAMB468 human triple-negative breast cancer (TNBC) and NIH3T3 mouse fibroblast cells have evinced their potential cytotoxic effect (75.98 ± 1.07) on cancer cells sparing the normal cells. Further, the results of the AO-EB dual staining assay indicated early apoptosis in TNBC cells by displaying greenish yellow-fluorescence in the nuclei at a IC50 concentration of 53.79 µg/mL of NPs. Additionally; the human erythrocyte membrane stabilization assay has corroborated the haemocompatibility of ZnOSA NPs by exhibiting a % HRBCMS up to 89.15 ± 0.02%. Furthermore, the photocatalytic experiments performed on Methylene Blue dye have revealed the excellent degradation efficiencies of ZnOSA NPs up to 88.01% with high stability and reusability tested during 4 catalytic cycles. Thus, the obtained results have shown greater potential in the anticancer and photocatalytic dye degradation activities of bio-fabricated ZnOSA NPs.