Theories of personality development highlight and emphasize the continuity between who we are as a child and who we are as an adult. However, there are major unanswered questions about this (dis)continuity because research on traits has long been divided into studies of temperament in childhood and studies of personality in adolescence and adulthood. To bridge this divide, the present study used longitudinal data from a sample of 674 Mexican-origin youth who completed assessments of temperament traits (i.e., negative emotionality, surgency, affiliation, effortful control) from ages 10-16 and assessments of Big Five personality traits from ages 14-26. Leveraging two waves of overlapping temperament/personality trait assessments at ages 14 and 16, we found (1) homotypic and heterotypic continuity between childhood and adolescent temperament and age 26 personality, with the strongest associations between homotypic traits and effortful control predicting all Big Five traits (except extraversion), suggesting self-regulation broadly promotes maturation; (2) temperament starts predicting homotypic adult personality traits by age 12-14, indicating adult personality begins to crystallize in adolescence; (3) homotypic temperament/personality reflect different expressions of the same underlying trait from age 10-26, established using latent growth models of joint temperament/personality factors; (4) mean-level personality development across late childhood to adulthood showing that all joint traits maintain consistent rank-order stability and youth increase in effortful control/conscientiousness, decrease in negative emotionality/neuroticism and surgency/extraversion, and do not change in affiliation/agreeableness. These findings add novel support for widely accepted–yet largely untested–theories, although some unexpected results undermine prevailing assumptions about personality development.