Anxiety and depression are pervasive and pernicious mental health problems for young adults. Developmental trajectories of adolescent temperament (Effortful Control, Negative Emotionality, Positive Emotionality) may help us predict who will experience anxiety/depression during young adulthood. The present study uses longitudinal data from a large, community sample of Mexican-origin youth (N = 674), to examine how temperament develops across adolescence (age 10 to 16) and whether the developmental trajectories of temperament are associated with anxiety/depression during young adulthood (ages 19 and 21). Results indicate that Effortful Control, Negatively Emotionality, and the Affiliation facet of Positive Emotionality tend to decrease across adolescence, whereas Surgency tends to increase. Greater increases in Effortful Control and Positive Emotionality across adolescence are associated with fewer anxiety/depression symptoms during young adulthood, whereas greater increases in Negative Emotionality are associated with more anxiety/depression symptoms later on. Thus, temperament development serves as both a protective factor (Effortful Control, Positive Emotionality) and a risk factor (Negative Emotionality) for later anxiety/depression in Mexican-origin youth.