Attention is critical for successful performance in demanding real-world situations. Yet, protracted periods of high demand may compromise attention and increase off-task thinking. Herein, we investigate if mindfulness training (MT) may promote cognitive resilience by curbing attentional lapses in high-stress cohorts. Two military cohorts were recruited during their high-stress predeployment interval. Mindfulness-based Mind Fitness Training (MMFT)® was provided to one group (MT, N = 31) but not the other group (military control group, MC, N = 24). The MT group attended an 8-week MMFT® course and logged the amount of out-of-class time spent practicing formal MT exercises. The Sustained Attention to Response Task (SART) was used to index objective attentional performance and subjective ratings of mind wandering before (T1) and after (T2) the MT course. In the MT group, changes in SART measures correlated with the amount of time spent engaging in MT homework practice, with greater objective performance benefits (indexed by A′, a sensitivity measure), and reduced subjective reports of mind wandering over time in those who engaged in high practice vs. low practice. Performance measures in the low practice and MC groups significantly declined from T1 to T2. In contrast, the high practice group remained stable over time. These results suggest that engaging in sufficient MT practice may protect against attentional lapses over high-demand intervals. Based on these results, we argue that MT programs emphasizing greater engagement in mindfulness practice should be further investigated as a route by which to build cognitive resilience in high-stress cohorts.