Gang-involved youth are disproportionately involved in criminal behavior, especially violence. The processes accounting for this enhanced illegal activity, however, remain speculative. Employing a life-course perspective, we propose that gang membership can be conceptualized as a turning point in the lives of youth and is thus associated with changes in emotions, attitudes, and routine activities, which, in turn, increase illegal activity. Using prospective data from a multisite sample of more than 1,400 youth, the findings suggest that the onset of gang membership is associated with a substantial change in emotions, attitudes, and social controls conducive to delinquency and partially mediate the impact of gang membership on delinquent activity. Desistance from gangs, however, was not associated with similar systematic changes in these constructs, including delinquent involvement.