A growth mindset is the belief that personal characteristics, specifically intellectual ability, are malleable and can be developed by investing time and effort. In recent decades, numerous studies have investigated the associations between growth mindset and academic achievement, and large intervention programs have been established to train adolescents to develop a stronger growth mindset. However, methodological research on the adequacy of the measures used to assess growth mindset is scarce. In our study, we conducted one of the first comprehensive assessments of the psychometric properties (especially structural and external validity, reliability, and measurement invariance across age groups) of Dweck’s widely used three-item Growth Mindset Scale in two samples: adolescents (age 14–19 years) and adults (age 20–64 years). Furthermore, we identified and validated a single-item measure to assess growth mindset in research settings with severe time constraints. Our results show that both the short (three-item) and ultra-short (single-item) scales have acceptable psychometric properties. However, associations with sociodemographic characteristics, personality characteristics, and achievement outcomes were generally small in the present samples. Further, we empirically demonstrated the comparability (i.e., measurement invariance) of the scales across adolescents and adults. Our findings can serve as a benchmark for future studies on growth mindset.