This study uses longitudinal data from the UK Millennium Cohort Study (N = 13,277) to examine the childhood and early adolescence factors that predict weapon involvement in middle adolescence, which in this study is exemplified by having carried or used a weapon. It finds that childhood experiences of low family income and domestic abuse between parents predict weapon involvement at age 17 years. Other predictors include childhood externalizing problems and self-harm in early adolescence. Further early adolescent behaviors and experiences that predict weapon involvement are own substance use, peer substance use, school exclusion, and high levels of electronic gaming. These findings provide concrete areas for targeting risk factors both in childhood and the early adolescent period, with an indication that early intervention and prevention are likely to reduce the need for later action.