Experiences with parents and romantic partners during adolescence are theorized to have long-term effects on youth development. However, little research has empirically examined the relative contributions of experiences in each type of relationship at different points during adolescence to positive development in young adulthood. The goal of the present study was to predict relative changes in youth positive personality characteristics, relational competence, and functional independence during young adulthood from specific behaviors experienced from parents and romantic partners during early and late adolescence. A diverse community sample of 147 individuals (59 males, 88 females) from the southeastern United States was repeatedly assessed across a 14-year period from age 13 to age 27. As hypothesized, parental acceptance and successful parental positive influence behavior toward adolescents at age 13 predicted relative increases in positive personality traits (e.g., agreeableness, conscientiousness, emotional stability) between ages 23 and 27. These same parental behaviors measured at age 18 were less robust predictors of key outcomes relative to similar qualities of romantic relationships. Instead, romantic behaviors of toleration and appreciation at age 18 predicted relative increases in functional independence and relational competence between ages 23 and 27 (e.g., attachment closeness, reliable alliance, nurturance, and functional independence). Results suggest that parents’ successful efforts to positively influence and accept their children during early adolescence may lay a foundation for future positive personality growth, and that similar positive behaviors experienced in late adolescent romantic relationships may help prepare youth to develop broader supportive social relationships and independence skills in young adulthood.