Changes in personality during adolescence may shape future alcohol use, especially in times of societal crisis. The COVID-19 pandemic disrupted social norms and stress regulation, making it a natural experiment to examine how early-life personality trajectories predict later substance use. Using longitudinal data from the European IMAGEN cohort (N = 968), we investigated whether intra-individual personality changes during adolescence predicted alcohol use at three timepoints: pre-pandemic, immediately after COVID-19 onset (T1), and several months into the pandemic (T2). Generalized Additive Mixed Models (GAMMs) were used to model non-linear relationships between adolescent personality change (e.g., in anxiety sensitivity and openness) and alcohol use frequency across time. Participants were drawn from the IMAGEN cohort (N = 968; 54.4% female), assessed longitudinally from early adolescence (BL: M = 13.93, SD = 0.41) through middle (FU1: M = 16.04, SD = 0.60) and late adolescence (FU2: M = 18.41, SD = 0.65) to early adulthood (FU3: M = 22.00, SD = 0.66), with additional follow-ups during the COVID-19 pandemic (COVID-T1: M = 25.00, SD = 0.81; COVID-T2: M = 25.56, SD = 0.75). Assessments were conducted at approximately 2-year intervals during adolescence and at shorter intervals (several months) during the COVID-19 period. Analyses controlled for sex and country-level lockdown stringency. Intra-individual fluctuations in anxiety sensitivity significantly predicted increased alcohol use several months into the pandemic (T2; EDF = 1.86, p < 0.05), but not immediately after lockdown (T1). Pre-pandemic alcohol use was instead associated with changes in openness (EDF = 1.00, p < 0.05). Exploratory analyses suggested gender differences in anxiety-related drinking risk. Our findings suggest that personality dynamics in adolescence shape alcohol use responses during societal crises. Different traits appear to matter depending on the temporal and emotional context of the stressor. These findings have implications for personalized prevention efforts targeting stress-related substance use in youth.