This study aimed to (1) evaluate how population levels of anxiety and depression grow and correlate from middle childhood through early adulthood, and (2) determine whether sex, family socioeconomic status, parental education, academic achievement, learning disabilities, or externalizing symptoms predict anxiety and/or depression levels and growth trajectory. We used two longitudinal samples (N = 445, 448) of Portuguese children. Mean depression levels increased from mid-childhood through adolescence before stabilizing in early adulthood and were most strongly predicted by academic achievement and learning disabilities. Mean anxiety levels increased until adolescence before decreasing across early adulthood and were most strongly predicted by academic achievement, learning disabilities, and externalizing symptoms. Quadratic models of growth fit best for both depression and anxiety, and depression and anxiety growth trajectories were strongly correlated. Though anxiety and depression trajectories differ in pattern and predictors, the two are highly interrelated and pathways to comorbid anxiety and depression should be characterized.