The integration of biochar as a sustainable reinforcement in cementitious composites has emerged as a dual-benefit strategy for carbon sequestration and biomass waste management in the construction industry. However, excessive biochar incorporation often compromises the mechanical properties of cement matrices. This study introduces an innovative, eco-friendly approach by synergistically combining biochar (2 %–10 % cement replacement) with recycled carbon fibers (1 %–3 %) to develop high-performance, low-carbon cement mortars. Experimental findings reveal that while the biochar addition reduces fluidity and prolongs setting times, it enhances the cement hydration degree through nucleation effects, internal curing, and accelerated CO₂ diffusion, promoting the formation of ettringite, calcium silicate hydrate gels, and the carbonation process. Despite improvements in flexural strength, biochar-induced porosity negatively impacts compressive strength. The incorporation of recycled carbon fibers counteracts this limitation, significantly enhancing the compressive and flexural strengths of biochar-augmented mortars by up to 14 % and 62 %, respectively, compared to plain cement matrices, through fiber bridging mechanisms. Although a strong interfacial bond is observed between the fibers and the biochar-modified matrices, the reinforcing efficiency is diminished with higher biochar dosages. Furthermore, the fiber and biochar contents should be limited to specific amounts to avoid excessive porosity and the resulting strength loss. Additionally, incorporating 10 % biochar alone into cementitious composites (10BC) provides the most favorable balance of environmental and economic performance under compressive strength criteria, achieving an emission reduction of 3.7 kg CO₂ eq./m³/MPa and a net cost of 2.4 CNY/m³/MPa. Further enhancement is possible by adding 2 % recycled carbon fiber to the 10BC mortar, increasing the total emission reduction to 41.1 kg CO₂ eq./m³/MPa when both compressive and flexural strengths are considered—albeit with an increase in economic costs. This study not only advances the development of low-carbon, high-strength building materials but also paves the way for the scalable application of biochar in the construction sector.