The objectives of this study were to model growth in anxiety and depressive symptoms from late school age through young adulthood in individuals with autism spectrum disorder (ASD) and controls with developmental delay (DD), and to assess relationships among internalizing growth patterns, participant characteristics, baseline predictors, and distal outcomes.Data were collected between ages 6 and 24 years in 165 participants (n = 109 with ASD; n = 56 with nonspectrum DD), most of whom received diagnostic evaluations in both childhood and early adulthood. Questionnaires were collected approximately every 3 to 6 months between ages 9 and 24 years. Parent-rated Child Behavior Checklist (CBCL), Adult Behavior Checklist (ABCL), and Developmental Behaviour Checklist anxiety- and depression-related subscale distributions were modeled with mixed-effects Poisson models, covarying diagnosis, age, verbal IQ (VIQ), gender, and significant 2- and 3-way interactions.Anxiety was positively associated with VIQ, and controlling for VIQ, both anxiety and depressive symptoms were greater in ASD than nonspectrum participants. Female gender predicted greater increases over time in anxiety and depressive symptoms for both diagnostic groups. Lower maternal education was associated with increasing internalizing symptoms in a subset of less verbal individuals with ASD. In exploratory post hoc analyses, internalizing symptoms were associated with poorer emotional regulation in school age, and with lower life satisfaction and greater social difficulties in early adulthood.Findings support previous claims that individuals with ASD are at particular risk for affect- and anxiety-specific problems. Although symptom levels in females increase at a faster rate throughout adolescence, males with ASD appear to have elevated levels of depressive symptoms in school age that are maintained into young adulthood.