This study employs radiocarbon measurements (Δ14C) to detect fossil fuel CO2 (CO2ff) in the megacity Delhi from 2018 to 2022, offering unique insights into fossil fuel emissions. Our measurements reveal varying Δ14C values ranging from −56.2‰ to 7.5‰ and CO2ff values ranging from 0.3 ppm to 25.2 ppm. We observed a distinct seasonality in Δ14C and CO2ff, with higher CO2ff (lower Δ14C) values during winter and post-monsoon seasons and lower CO2ff (higher Δ14C) in summer and monsoon seasons. We have determined emission ratios (RCO and RNOx), and compared them with other studies worldwide. Our study also examines the impact of the COVID-19 lockdown periods on CO2ff and observed a notable decrease of 5.6 (4.8) ppm during the pandemic’s first (second) waves. These findings enhance our understanding of CO2 emission inventories in megacity Delhi.