The timing of seasonal biological events—such as when trees leaf out, flowers open, and leaves turn yellow—is called phenology. Plant phenology is sensitive to climate change and variability; however, how complex urban environments affect plant phenology remains largely unexplored. Here we investigated how phenology responded to urban warming and artificial light in large U.S. cities using satellite data and phenology models. We found spring green-up occurred 6 days earlier but tended to be less sensitive to temperature in urban areas, compared to rural areas. Phenology models show that the earlier green-up is due to urban warming and the reduced sensitivity is likely caused by insufficient chilling in winter. We further show artificial light at night shifts spring phenology 9 days earlier and autumn phenology 6 days later on average, with large regional variation. As a result, cities have a longer growing season, which consequently affects pollen allergy season, water cycling, and plant-pollinator matchup. The findings contribute to a better understanding of urban ecosystem changes under climate change and anthropogenic impact to inform more ecologically sensitive urban planning, design, and management.