--- - |2+ Objective This study investigated the reliability and factor structure of the General Health Questionnaire‐12 (GHQ‐12) in children and adolescents and examined whether the GHQ‐12 is sensitive to expected mental well‐being differences across age and sex. Method Here, N = 180,700 Australian students (7–19 years of age) completed the GHQ‐12 as part of a larger survey, the Resilience Survey (Resilient Youth Australia Limited). Exploratory factor analysis (EFA) was conducted, and internal consistency was assessed with Cronbach's alpha. Linear mixed model ANOVAs were conducted to investigate differences in GHQ‐12 scores between females and males. Results EFA revealed a two‐factor model which was consistent across all age bands—Factor A: General Dysphoria (depression and anxiety), and Factor B: General Functioning (ability to cope with day‐to‐day activities). Internal consistency was good (Cronbach's α > 0.7) in all age bands for total GHQ‐12 and factor scores. Confirmatory factor analysis with a two‐factor correlated structure supported EFA results. Bifactor modelling suggested a unidimensional structure. Males aged 7–9 years had significantly higher (more problematic) total GHQ‐12 scores, General Dysphoria scores and General Functioning scores than females (p < 0.001), and females aged 12–19 years had significantly higher scores than males (p < 0.001). Conclusions Our results support the use of the GHQ‐12 for the measurement of mental well‐being symptoms in children from 7 to 19 years of age. Overall, psychometric properties including sensitivity, suggest that the GHQ‐12 provides a robust indicator of short‐term mental state in children and adolescents. - Australian Journal of Psychology, EarlyView.