Despite the robust link between dispositional optimism and well-being across the lifespan, the developmental origins of dispositional optimism are unknown. Understanding the pathways that lead to greater optimism during the transition from adolescence into young adulthood may be important given that this stage of the life course involves the navigation of multiple simultaneous psychosocial demands. Maternal attachment security may contribute to greater optimism by promoting perceptions of internal control. Participants were 218 European American children (98 females; 120 males), who completed self-report measures across four waves. A path analysis tested the associations between maternal attachment security (ages 10 and 14), locus of control (age 14), dispositional optimism (ages 18 and 23), and psychological well-being (age 23). Tests of indirect effects showed that greater perceptions of internal control at age 14 mediated the association between age 10 maternal attachment security and age 18 dispositional optimism. Age 18 dispositional optimism mediated associations between age 14 internal control and age 23 psychological well-being. Maternal attachment security may promote dispositional optimism through a greater internal locus of control in adolescence. Given that optimism promotes well-being throughout the lifespan, identifying the pathways through which optimism develops may contribute to understanding how to promote well-being.