Application of carbon-negative biochar in cement composites can effectively reduce carbon emissions of construction materials. However, strength reduction would be inevitable for the cement composites after the incorporation of high-volume porous biochar. Carbonated enhancement of interfacial transition zone (ITZ) in biochar-cement composite was developed by utilizing engineered biochar and CO₂ curing to compensate for the strength degradation. The impact of biochar customized with different temperatures and activation methods on facilitating cement carbonation was explored. The increase of pyrolysis temperature and/or the application of CO₂ or steam activation for biochar preparation could increase water absorption rate and CO₂ adsorption capacity during subsequent CO₂ curing, thus promoting the degree of cement hydration and carbonation. Carbonated ITZ mainly composed of CaCO₃ with an elastic module range of 15–25 GPa would generate and surround the customized biochar (e.g., biochar prepared at 750 °C with CO₂ activation). The results show that the samples with 10 wt% biochar incorporation under CO₂ curing could achieve lower density and comparable compressive strength with the pure cement systems that have a limited carbonation level. Our novel design of customized biochar and CO₂ curing could compensate for the strength deficiency of biochar-cement composites and extend the emerging applications of porous biochar in the cementitious composites for low-carbon construction.