Although Mexican-origin youth with first-generation immigrant parents are relatively good at retaining their heritage language of Spanish, limited research has been conducted on their Spanish language development during adolescence. From three-wave longitudinal data across six years (Nwave1 = 604, Mage.wave1 = 12.91, 54% female), distinct groups of adolescents with consistently high, improved, declined, and consistently low Spanish proficiencies were identified. Family relationship quality was more predictive of adolescents’ Spanish proficiency than family language environment. The benefits of Spanish proficiency were consistent across adolescents’ ethnic identity, resilience, and life meaning. More research and practical attention to parent–adolescent relationships is needed to capitalize on the continued plasticity of adolescents’ Spanish language development and to promote consequent positive outcomes.