The present study was meant to investigate longitudinal change in pro-environmental behaviour from the early teenage years to early adulthood as it relates to changes in nature connectedness and the endorsement of proenvironmental norms. A cross-sequential study design was used in which two cohorts of Canadian adolescents (12- to 14-year-olds, n = 220, 110 females, and 18- to 20-year-olds, n = 390, 305 females) were followed up longitudinally over four years and three waves of data collection. All measures were based on Rasch scales. Latent growth models demonstrated non-linear change in both cohorts. Pro-environmental behaviour, connectedness with nature and norm endorsement significantly increased in the first two years of the longitudinal study, while this increase levelled off thereafter. Cross-lagged panel analyses demonstrated that nature connectedness and endorsement of pro-environmental norms reciprocally predicted pro-environmental behaviour in both cohorts, with one dominant path from pro-environmental behaviour to norm endorsement. As adolescents and young adults continue to engage in pro-environmental behaviour, they increasingly tend to endorse pro-environmental norms. Connectedness with nature and norm endorsement, by contrast, did not evidence any cross-lagged relationship. Overall, the study provides valuable insights into how the disposition to engage in pro-environmental behaviour forms in youth.