Pure CeO2 and Ce1‐xMnxO2, where x = 0, 2, 4 and 6% w/w, were synthesized by sol‐gel method using Acacia Concinna fruit extract as a surfactant and stabilizing agent. The synthesized particles were subjected to various characterization techniques, including XRD, HRTEM, FESEM, FTIR, Raman and UV‐Visible spectroscopy. The XRD pattern of the samples demonstrated single phase of CeO2, with cubic fluorite type structure and crystallite size of 6–7 nm. The morphology of the nanoparticles (NPs) was analyzed by FESEM and HRTEM which illustrated an aggregate of irregular spherical nanoparticles and the average particle size is around 18 nm. Raman spectroscopy validated the phase purity and indicated about the structural defects caused by the dopants. The bandgap energy of CeO2 NPs is calculated to be 3.00 eV from the Tauc plot of UV‐Visible spectroscopy, which gradually decreased to 2.43 eV with increase in manganese doping. Cytotoxicity studies signified the role of Mn‐doped CeO2 NPs as a potential nanotherapeutic agent for cancer treatment. The cell viability assay showed that 200 µg/mL of 6% Mn‐doped CeO2 nanoparticles exhibited remarkable cytotoxic effect by inhibiting 50% of cancer cells both in breast and colon cancer cell lines without affecting the healthy cell lines.