Display omitted •Green synthesis of transition metal (Co, Cu, Zn) doped magnesium oxide nanoparticles.•The fcc nanocrystals have average crystallite sizes ranges from 5.89 to 9.99 nm.•Enhanced catalytic efficiency of Co and Cu-doped MgO NPs has been observed.•Samples exhibited antibacterial and antioxidant activities.•Synthesized NPs are potential agents for treating wastewaters. This study focuses on the fabrication of transition metal (Co, Cu, Zn) doped magnesium oxide nanoparticles (MgO NPs) using mint plant leaf extract. The resulting samples were subjected to thorough evaluations of their photocatalytic, antibacterial, and antioxidant properties. UV–Visible studies revealed distinct band gap energies for pure and doped MgO NPs: 3.94, 1.76, 1.99 and 4.05 eV respectively. Analysis of X-ray diffraction (XRD) spectra established the presence of face centered cubic (fcc) nanocrystals with average crystallite sizes ranges from 5.89 to 9.99 nm. The incorporation of dopants, Co, Cu, and Zn into the MgO NPs was verified through energy dispersive X-ray (EDX) analysis. Fourier transform infrared (FTIR) results indicated modifications in the Mg-O stretching frequency due to the doping process. Furthermore, transmission electron microscopy (TEM) investigations unveiled particle sizes varied from 19 to 30 nm. Photocatalytic activity assessments underscored the enhanced catalytic efficiency of Co and Cu-doped MgO NPs. The Cu doped MgO NPs exhibited superior performance towards gram-positive bacteria (B. subtilis and S. aureus) relative to gram-negative bacteria (E. coli) in antibacterial studies. Antioxidant activity, assessed using the DPPH assay, revealed scavenging rates of 58–62 %. Overall, these findings suggest that the synthesized NPs hold substantial promise as potential agents for treating wastewater contaminated by dyes, pathogens, and free radicals.