Intracellular hydrogen peroxide levels have the potential to be exploited in cancer therapy. We have synthesized uniform and size-controlled copper-impregnated mesoporous silica nanoparticles (Cu–MSN) containing a catalase inhibitor, 3-amino-1,2,4-triazole (AT) for ROS-mediated apoptosis in cancer cells. Copper species in the framework and aminotriazole loading were confirmed by electron spin resonance (ESR) and nuclear magnetic resonance (NMR) spectroscopies, respectively. The metal–ligand binding between the copper and AT are sensitive to the endosomal environment (pH = 5.5) for the release of AT to inhibit cytosolic catalase activity. The subsequent enhanced level of intracellular hydrogen peroxide, after catalase inhibition, is transformed into toxic, reactive oxygen species (ROS) by the catalysis of Cu(II) on MSN. The intracellularly delivered Cu–MSN–AT exhibited significant activity against colon carcinoma (HT-29 cell line) substantiated by increased levels of ROS, where AT drove up the hydrogen peroxide concentration and also the level of free radicals through a Fenton-like reaction. The trigger of cell apoptosis was induced from ROS attack to lipid membranes in order for further radical propagation to cause lipid peroxidation and eventually, a decrease in the membrane fluidity and mitochondrial membrane potential. As a result, an increase in the membrane permeability caused the release of cytochrome c into the cytoplasm and further activation of apoptotic cascades to trigger DNA fragmentation. The design of the Cu–MSN–AT system takes advantage of a synergistic effect to inhibit antioxidant defenses and catalyze the activation of lethal ROS by the framework of copper ions to kill cancer cells which represents a novel chemotherapeutic strategy for ameliorating the toxic side effects from non-specific ROS generation in traditional chemotherapeutic agents.