This study presents an innovative approach to sustainable construction by utilizing stubble waste biochar as a cementitious composite material. Crop stubble burning is a critical environmental issue, contributing significantly to air pollution and carbon emissions. This research addresses this challenge by converting stubble waste into biochar, a carbon-rich material with substantial potential for carbon sequestration and construction applications. Comprehensive characterization of biochar revealed key properties, including a fine particle size (1.23 μm), complex pore structure for improved water absorption and retention, and the presence of silica and calcite phases, which contribute to enhanced cementitious behavior. Biochar, produced through the pyrolysis of rice stubble, was incorporated into mortar mixes at replacement levels of 0%, 5%, and 10% by weight of cement. The mixes underwent carbonation curing to enhance carbon capture and were evaluated for workability, compressive strength, flexural strength, and water absorption. While biochar incorporation initially reduced workability due to high water absorption, this was mitigated by adjusting superplasticizer dosages. Post-carbonation curing, the compressive strength of biochar mixes increased by 20.5% and 35.3% for 5% and 10% biochar, respectively. Flexural strength improvements were even more pronounced, with increases of 68.7% and 102.5% for the same biochar levels. Additionally, water absorption decreased by 9.9% and 5.2% for 5% and 10% biochar mixes, respectively. These results highlight the dual benefits of biochar in enhancing mechanical properties and contributing to carbon sequestration, showcasing its promise as a sustainable and carbon-negative material for construction applications.