The global economy faces serious socio-economic challenges due to environmental pollution and the proliferation of antibiotic-resistant pathogenic microorganisms. Consequently, nanomaterial partially of metallic nature have been synthesized with both catalytic and therapeutic propertiesProperties to inhibit the activities of harmful bacteria and also degrade organic pollutant particularly dyes in wastewater. In this study, the antibacterialAntibacterial and photocatalytic potential of biogenic MgONPsMagnesium oxide nanoparticles (MgONPs) using aqueous leaf extracts of agricultural biomassBiomass from Hevea brasiliensis (rubber tree), Chromolaena odorata (Siam weed), and Elaeis guineensis (African oil palm) were examined. The biochemicals present in this agricultural wasteWastes served as reducing and capping agents, thereby facilitating the synthesisSynthesis of the nanoparticlesNanoparticle. The samples were analysed using FTIR, SEMScanning Electron Microscopy (SEM), DLS, XRDX-Ray Diffraction (XRD), and BET revealing nanoscale crystallinity, mesoporosity, and size distributions ranging from 44 to 100 nm. SWL-mediated MgONPsMagnesium oxide nanoparticles (MgONPs) exhibited the smallest size, largest surface area (355.9 m2/g), and demonstrated superior stability among the synthesized materials. AntibacterialAntibacterial assays against Escherichia coli and Staphylococcus aureus demonstrated strong inhibition, with Siam weed-MgONPsMagnesium oxide nanoparticles (MgONPs) showing activity comparable to ciprofloxacin. Photocatalytic evaluation confirmed efficient degradation of methylene blue dye under sunlight, with maximum degradation (92.8%) and mineralization (99.1%) achieved by Siam weed-MgONPsMagnesium oxide nanoparticles (MgONPs). These findings establish valorization of underutilized tropical biomassBiomass for low-cost, sustainable synthesisSynthesis of multifunctional nanomaterials applicable in wastewater treatment and antimicrobial applications.