Urban greenland plays a vital role in carbon storage, yet its impact across spatial scales remains understudied. This study investigates Nanjing’s greenland dynamics and their influence on carbon storage from 2000 to 2020, using LUCC, elevation, climate, and socio-economic data. By integrating the InVEST model and Geodetector tool, the results show that: (1) Greenland area decreased by 4.84%, with increased landscape fragmentation. (2) Carbon storage declined from 12.17 to 11.3 t/hm2, showing spatial variation—higher in peripheral greenland and cropland, lower in urban centers. (3) Land conversion significantly affects carbon storage, with the largest losses from greenland to unused land, and greatest gains in reverse. (4) Elevation, temperature, and GDP are key drivers, with elevation showing the strongest individual effect (q = 0.263) and its interaction with GDP the highest (q = 0.336). The study suggests optimizing greenland layout, increasing urban parks, and conserving peripheral greenland to enhance carbon sinks and support sustainable urban development.