Background Young people exposed to violence are at increased risk for mental health and behavioral problems. However, very little is known about the immediate, or same‐day, associations between violence exposure and adolescents' mental health symptoms or whether daily symptom or behavioral reactivity marks future problems. Methods Young adolescents were assessed three times a day for 30 consecutive days using mobile‐phone‐based Ecological Momentary Assessment (EMA) (N = 151 adolescents). Over 12,500 assessments and 4,329 person days were obtained via the EMA. Adolescents were recruited from low‐income neighborhoods based on parent‐reported risk for externalizing symptoms. Mental health symptoms were assessed via parent and child report at baseline, multiple times per day via EMA assessments of the adolescents, and again 18 months later when 93% of the adolescents were reinterviewed. Results Results from multilevel models illustrated that young adolescents were more likely to experience symptoms of anger (OR = 1.74, CI: 1.31–2.30), depression (OR = 1.66, CI: 1.26–2.19), and conduct problems (OR = 2.63, CI: 1.71–4.04) on days that they were exposed versus not exposed to violence. Increases in depressive symptoms were also observed on days following violence exposure (OR = 1.46, CI: 1.09–1.97). Adolescents with the highest levels of violence exposure across the 30‐day EMA were less behaviorally reactive to violence exposures in daily life, and heightened behavioral reactivity predicted increased risk for substance use across early adolescence. Conclusions Findings support the need to focus on both the immediate and long‐term associations between violence exposure and adolescents' mental health and behavior. Results also suggest that heightened behavioral reactivity during early adolescence may signal emerging substance use problems.