The effect of the injection of externally sourced carbon dioxide (CO2) on the stability of the flameless combustion regime was evaluated numerically and experimentally, taking temperature uniformity and pollution emissions (NO and CO) as criteria. The flameless combustion regime was studied in a lab-scale furnace fueled with natural gas (NG) at a thermal power of 20 kW based on the low heating value (LHV). The CO2 was injected into the lower part of the furnace to directly affect the reaction zone. Computational fluid dynamics (CFD) simulations were performed using the ansys-fluent software. The models used to describe the turbulence, the radiation heat transfer, and the turbulence–chemistry interaction were the standard k–ɛ model, discrete ordinate model (DOM), and eddy dissipation concept (EDC) model, respectively. The NG oxidation was described with a seven-step global reaction mechanism with the EDC model. Three excess air conditions were analyzed, 20%, 25%, and 30%, combined with various CO2 injection flows. At 30% excess air, the flame exhibited destabilization without any CO2 injection. Adding CO2 attenuates the destabilization because of the dilution effect. Increasing either the CO2 or excess air flow resulted in a considerable decrease in the global temperature of the process, consequently producing an increase in CO emissions and a decrease in NO emissions. Finally, for the conditions studied, increasing the mass flow of externally sourced CO2 did not destabilize the flameless combustion regimen. This result shows the potential of the implementation of flameless combustion in industrial processes where CO2 is releasing as a result of a reaction external to the combustion process, such as cement, ammonia, or lime production among others.