This article used data from 5,171 young women participating in the Australian Longitudinal Study on Women’s Health, a nationally representative longitudinal cohort study, to identify longitudinal trajectory patterns of mental health across 6 surveys over 16 years of early adulthood, from age 18–23 to age 34–39. In addition, we identified both predictors and outcomes of these trajectories. Using group-based trajectory modeling, we identified 4 distinct trajectory groups of mental health. The mental health of most participants (55%) was consistently high, with 12% improving, 24% varying, and 9% frequently low. The authors considered characteristics at the beginning and end of the trajectory period, taking a life-course perspective to understand vulnerabilities to, and outcomes of, low or variable poor mental health trajectories. Financial difficulties, poor general health, and weight or shape dissatisfaction were characteristics at Survey 1 that distinguished all other trajectory groups from those with consistently high mental health. Other differences were specific to 1 or 2 groups. By the end of the trajectory period, the improving mental health group showed few differences from those with consistently high mental health. However, those with varying and low mental health showed evidence of social disadvantage, poor physical and emotional health, and unhealthy behaviors, and were less likely to be mothers. The ability to identify distinct trajectories of mental health in early adulthood, and their correlates, provides evidence to underpin population health interventions targeting the prevention of mental health problems among this population group.