In this study, we investigated the effect of size variation in CeO2/MgFe2O4 nanocomposites (NCs) on their electrochemical (EC) performance and cytotoxicity. To adjust particle size, the NCs were synthesized using an auto-combustion process and subsequently at various (500, 600, 700, and 800 °C) calcined temperatures (CTs). The crystallite size was determined using X-ray diffraction (XRD), and the NCs spherical shape was validated using field emission-scanning electron microscopy (FE-SEM) and transmission electron microscopy (TEM). The chemical components on the surface of the NCs were discovered using an X-ray photoelectron spectroscopy (XPS). The different size distributions of CeO2/MgFe2O4 NCs as electroactive materials in supercapacitors and measured their EC performance in 1 M KOH using cyclic voltammetry (CV), galvanostatic charge/discharge (GCD) and electrochemical impedance spectroscopy (EIS). The CeO2/MgFe2O4 NCs revealed a good cell survival rate in cytotoxicity studies, showing their biocompatibility. The NCs were found to be harmless to HDF and MDA-MB-231 cancer cell lines, highlighting their potential for biological applications in the future.