Supercritical CO2 was utilized to accelerate the carbonation process, altering the microstructure and composition of Portland cement mortar blended with metakaolin. Mortar samples were cured in a high-pressure reactor at 323 K and 15 MPa under supercritical CO2 conditions. The results demonstrate a significant enhancement in both compressive and flexural strength for all carbonated mortar samples treated with supercritical CO2 compared to those subjected to ambient curing conditions. Furthermore, longer reaction times resulted in increased overall CO2 utilization within the mortar matrix, with more pronounced effects observed in samples containing metakaolin. Hydrated mortar samples demonstrated a complex pore system characterized by a substantial presence of small gel pores, alongside large gel and mesopores. Upon exposure to supercritical CO2, SEM micrographs revealed a rougher surface, along with the formation of CaCO3 crystals. Comprehending high-pressure carbonation is vital for several applications, such as carbon capture and storage, improving the properties of recycling concrete, and safely managing toxic elements in construction waste materials.