This nine-year longitudinal study addresses the joint contribution of parent-rated negative controlling parenting and child personality on psychosocial outcomes in 141 families of children with autism spectrum disorder (83% boys, mean age Time 1 = 10.1). Latent change modeling revealed substantial variation in within-person change in parenting and psychosocial outcomes across a six- and three-year-interval. Over time, negative controlling parenting and child personality were consistently related to externalizing problems, whereas child personality was differentially related to internalizing problems and psychosocial strengths. Three personality-by-parenting interactions were significant, suggesting that children with less mature personality traits show more externalizing behaviors in the presence of controlling parenting. This study identified both parenting and child personality as important modifiers of developmental outcomes in youth with autism.