Ferrous-cobalt (Fe-Co) co-doped tin oxide (SnO2) nanoparticles (NPs) were prepared using Psidium guajava leaf extract by microwave-assisted green synthesis. FeCo:SnO2 NPs was characterized using Fourier transform infrared spectroscopy (FTIR), Powder X-ray diffraction (PXRD), Field emission scanning electron microscopy (FESEM) and Energy dispersive X-ray spectroscopy (EDAX) techniques. PXRD patterns revealed the tetragonal structure of FeCo:SnO2 NPs, with an average crystalline size of 67 nm. The Sn-O stretching bands were detected in the FTIR spectra at 441 cm-1. FESEM analysis showed that FeCo:SnO2 NPs had a spherical structure and chemical composition of the elements such as Fe, Co, Sn and O identified by EDAX spectra. The oxygen vacancy of surface defects was explored using photoluminescence (PL) spectroscopy. The antibacterial activity of prepared NPs was evaluated against Staphylococcus aureus (S.aureus) and Klebsiella pneumonia (K. pneumonia) using well diffusion method. FeCo:SnO2 NPs exhibited better antibacterial activity than amoxicillin. The anticancer activity of FeCo:SnO2 NPs was explored against MCF-7 cell lines (human breast cancer). The biocidal results demonstrate that Psidium guajava leaf extract-derived FeCo:SnO2 NPs showed significant inhibition against tested bacterial strains and enhanced potency against MCF-7 cancer cell lines.