This work uses Psidium guajava leaf extracts to easily and quickly synthesise Cerium nanoparticles (Ce NPs) and Zn: CeO2 NPs (Zn-Ce NPs) (PGE). The generated G - CeO2 and Zn: CeO2 NPs were examined using X-ray diffraction (XRD), Energy Dispersive Analysis of X-Ray (EDAX), Fourier transformed infrared spectroscopy (FT-IR), UV-Visible spectroscopy (UV-Vis), and Scanning Electron Microscopy (SEM) techniques. The presence of OH, -NH2, and C=O groups, which were essential for the synthesis of G-CeO2 and Zn: CeO2 NPs, was detected by FT-IR spectroscopy. The XRD analysis supports the fluorite cubic structure of the resulting particles for G-CeO2 and Zn: CeO2 NPs. The findings of the elemental analysis by EDX demonstrated that the doping effects of Zn2+ in G-CeO2 NPs effectively enhanced the percentages of cerium and oxygen. G-CeO2 and Zn: CeO2 nanoparticles were evenly dispersed, and a SEM microscopic investigation also revealed that the produced nanoparticles were spherical in shape. Both the G-CeO2 and Zn: CeO2 nanoparticles demonstrated effective antibacterial and antifungal properties. To measure photocatalytic activity, the degradation of methylene blue (MB) in aqueous solution under visible light irradiation was utilised. The findings demonstrated that Zn doping considerably increased Ce NPs' photocatalytic activity. Using Zn dopant, it has been discovered that the optical band gap of Ce NPs has decreased from 3.69 eV to 3.55 eV. Both the G-CeO2 and Zn: CeO2 nanoparticles demonstrated effective antibacterial and antifungal properties. By increasing the concentration of both G-CeO2 and Zn: CeO2 NPs, the viability of the cancer cells L929 and HCT 116 was partly reduced.