Young adulthood is a critical developmental life stage and a period of enhanced vulnerability to stress. In 2020, young adults in Northern California were faced with a series of unforeseen, collective stressors: the COVID-19 pandemic, extreme wildfires, social tension associated with the murder of George Floyd, and a contentious election that culminated in an attack on the nation's capital. In a natural experiment, we compared the psychosocial development of 415 young adults across 8 monthly assessment waves during 2020 to a control cohort (n = 465) who completed the same assessment protocol in 2019, prior to the onset of stressors. Results of latent growth curve models indicated that the 2020 cohort had less adaptive trajectories of affective well-being and lower levels and less adaptive trajectories of social functioning, suggesting detrimental effects of cumulative, collective stressors on the socio-emotional development of young adults.