Objective: Measurement reliability is assumed when executive function (EF) tasks are used to compare groups or to examine relationships between cognition and etiologic and maintaining factors of psychiatric disorders. However, the test–retest reliabilities of many commonly used EF tasks have rarely been examined in young children. Furthermore, measurement invariance between typically developing and psychiatric populations has not been examined. Method: Test–retest reliability of a battery of commonly used EF tasks was assessed in a group of children between the ages of 5 and 6 years old with (n = 63) and without (n = 44) ADHD. Results: Few individual tasks achieved adequate reliability. However, confirmatory factor analysis (CFA) models identified two factors, working memory and inhibition, with test–retest correlations approaching 1.0. Multiple indicators multiple causes (MIMIC) models confirmed configural measurement invariance between the groups. Conclusion: Problems created by poor reliability, including reduced power to detect group differences, index change over time, or to identify relationships with other measures, may be mitigated using latent variable approaches.