Tests of general aptitude - such as working memory capacity (WMC), fluid intelligence (gF), or standardized admissions tests (e.g. SAT; GRE) - are thought to capture variability in crucial cognitive capacities that are broadly predictive of success, yet pinpointing the exact nature of these capacities is an area of ongoing controversy. We propose that mind-wandering is associated with performance on tests of WMC and gF, thereby partially explaining both the reliable correlations between these tests as well as their broad predictive utility. Employing complementary methodological designs, Studies 1-4 embedded thought sampling into tests of general aptitude and determined that mind-wandering was consistently associated with worse performance. Collectively, Studies 1-4 implicate the capacity to avoid mind-wandering during demanding tasks as a potentially important source of success on measures of general aptitude. Accordingly, Studies 5-7 investigated whether mindfulness training would reduce mind-wandering and thereby enhancing performance on tests of general aptitude. The results indicate that brief mindfulness exercises can reduce mind-wandering in at least some task contexts. Furthermore, two-weeks of mindfulness training program improved WMC and reading comprehension, while simultaneously reducing the occurrence of distracting thoughts during these tasks.