Metacognitive biases have been repeatedly associated with transdiagnostic psychiatric dimensions of ‘anxious-depression’ and ‘compulsivity and intrusive thought’, cross-sectionally. To progress our understanding of the underlying neurocognitive mechanisms, new methods are required to measure metacognition remotely, within individuals over time. We developed a gamified smartphone task designed to measure metacognitive (confidence) bias and investigated its psychometric properties across two studies (N=3410 unpaid citizen scientists, N=52 paid participants). We assessed convergent validity, split-half and test-retest reliability, and identified the minimum number of trials required to capture its clinical correlates. Convergent validity of metacognitive bias was moderate (r(50)=0.64, p<0.001) and it demonstrated excellent split-half reliability (r(50)=0.91, p<0.001). Test-retest reliability was also very high (ICC=0.86, N=110). Anxious-depression was associated with decreased confidence (B=-0.23, SE=0.02, p<0.001), while compulsivity and intrusive thought was associated with greater confidence (B=0.07, SE=0.02, p<0.001). Metacognitive biases in decision-making are highly stable within-session, exhibiting strong reliability and associations with clinical correlates are evident in as few as 40 trials. Meta Mind exemplifies how to adapt established cognitive tasks for within-person, longitudinal assessments in computational psychiatry, facilitating the development and testing of causal models.