Radiocarbon (¹⁴C) serves as a unique physical tracer for fossil fuel CO₂ (CO_2ff) owing to its absence in ancient fuels. This review synthesizes methodologies and applications of ¹⁴C in quantifying CO_2ff emissions from urban to regional scales. It outlines the theoretical framework for partitioning CO_2ff from other sources using Δ¹⁴C and summarizes advances in sampling strategies and accelerator mass spectrometry (AMS) analysis. Key methodological challenges—including disequilibrium fluxes from terrestrial and oceanic reservoirs, sparse observational networks, and uncertainties in atmospheric inversion models—are critically assessed. The review highlights the pivotal role of ¹⁴C in independently verifying rapid, policy-driven emission reductions during the COVID-19 lockdowns, which provided a clear signal distinct from natural variability. Case studies, with a particular focus on China, demonstrate its utility in tracking spatial gradients and long-term trends. Looking forward, synergistic pathways that integrate multi-tracer observations, expanded monitoring networks, and enhanced modeling are discussed to strengthen the role of ¹⁴C within a comprehensive CO_2ff monitoring and verification framework.