This study presents a systematic literature review evaluating adaptive reuse and retrofitting of existing buildings as effective strategies for reducing carbon emissions and addressing climate change. Drawing from research published between 2020 and 2025, the review synthesizes quantitative findings on energy savings, embodied carbon reductions, and economic impacts across various building types, including cultural, residential, industrial, and educational structures. Case studies reveal that retrofitting measures—such as insulation upgrades, efficient glazing, and integration of solar systems—can reduce operational energy use by 65–86%, with some buildings achieving near-net-zero energy status and annual carbon dioxide emission reductions of up to 32.8 tons. The reuse of materials and the adoption of bio-based construction techniques are shown to reduce embodied carbon by as much as 92% and greenhouse gas emissions by up to 14%. Economic assessments indicate promising returns, with payback periods as short as 2.3 years and potential savings projected at €350 billion by 2030. The review highlights the importance of integrating life cycle assessment (LCA), building information modeling (BIM), and circular economy principles to enhance environmental performance. Despite the recognized benefits, challenges remain in regulatory alignment, performance benchmarking, and decision-making frameworks. The findings underscore the importance of adaptive reuse and retrofitting as vital components of sustainable development, offering significant opportunities to decarbonize the built environment and contribute to global climate change mitigation goals.