The study investigated a developmentally sensitive group, emerging adults going through a critical post-secondary educational transition in tandem with the first wave of the COVID pandemic in Finland. The participants (n = 330) were surveyed initially during their first year in secondary education, and again during the pandemic, right after their graduation in summer 2020. According to latent change score models, there was a small mean level increase in anxiety but interestingly, the increase was stronger among individuals with fewer prior anxiety symptoms and fewer prior experiences of loneliness. However, during-pandemic experiences of social inclusion and living with parents were protective against an increase in anxiety symptoms. Findings from this longitudinal study add to the understanding of the complexity of the COVID impact on mental health and the intertwined dynamics of social relations and mental well-being among emerging adults.