Biochar is a known product to permanently remove carbon from its cycle. It is essential to find high-quality and large-quantity utilization for biochar. This study assessed the efficacy of biochar on the hydration of magnesia cement (MC) and magnesia cement-Portland binary cement (MP)-based pastes and evaluated the synergistic effect of biochar and CO2 curing on the pastes. The thermogravimetric and X-ray diffraction analyses showed that the incorporation of biochar, especially CO2 gasification biochar, promoted the generation of hydration products due to the internal curing effect. The use of CO2 curing effectively accelerated the carbonation of pastes. Hydrated magnesium carbonates were preferentially formed in CO2-cured MC pastes, whereas CaCO3 was preferentially generated in CO2-cured MP pastes. Moreover, the incorporation of biochar, especially porous CO2 gasification biochar, could further facilitate CO2 diffusion and promote carbonation. As a result, the synchronous use of biochar and CO2 curing significantly enhanced the mechanical strength of blocks. Therefore, biochar-augmented and CO2-enhanced composites could be novel and low-carbon construction materials for sustainable engineering applications.