Biochar has been increasingly used in the production of cementitious materials due to its low cost, low-carbon emission, and environmental benefits. This study provides a comprehensive review on the effect of biochar on the performance of cementitious composites, focusing on mechanical properties, durability properties, and carbon-sequestration capacity. It has been observed that the use of biochar can improve the mechanical strength, thermal, and electromagnetic performance of hardened biochar-cement composites. The optimum cement replacement with biochar is 1–2 wt% (by weight) for enhancing the compressive and flexural strength. Additionally, the addition of biochar can improve the resistance to sulphate attacks, chloride-induced corrosion, shrinkage, and permeability of biochar-cement composites. Biochar also has the potential to reduce the permeability of concrete, and no significant differences were observed in permeability reduction for biochar processed at different pyrolysis temperatures. The positive effect of biochar (up to 5 wt%) on durability improvement is attributed to enhanced hydration and physical filling, resulting in a denser microstructure that prevents the penetration of ions and water. This study also discusses the impact of biochar on carbon sequestration capacity, demonstrating its ability to enhance the carbon-sequestration capacity of biochar-based concrete. In conclusion, while the mechanical properties of concrete with biochar have been extensively investigated, future research is needed to explore the long-term durability properties under different environmental conditions. Moreover, there is a growing demand for low-carbon concrete that utilizes carbon-negative materials to enhance performance and resilience.