BACKGROUND: Little is known about whether brief mindfulness ecological momentary interventions (MEMIs) yield clinically beneficial effects. This gap exists despite the rapid growth of smartphone mindfulness applications. Specifically, no prior brief MEMI has targeted generalized anxiety disorder (GAD). Moreover, although theories propose that MEMIs can boost executive functioning (EF), they have largely gone untested. Thus, this randomized controlled trial (RCT) aimed to address these gaps by assessing the efficacy of a 14-day smartphone MEMI (versus self-monitoring placebo [SMP]). METHOD: Participants with GAD were randomly assigned to either condition (68 MEMI and 42 SMP). MEMI participants exercised multiple core mindfulness strategies and were instructed to practice mindfulness continually. Comparatively, SMP participants were prompted to practice self-monitoring and were not taught any mindfulness strategies. All prompts occurred five times a day for 14 consecutive days. Participants completed self-reports and neuropsychological assessments at baseline, posttreatment, and 1-month follow-up (1MFU). Piecewise hierarchical linear modeling analyses were conducted. RESULTS: MEMI (versus SMP) produced greater pre-1MFU reductions in GAD severity and perseverative cognitions (between-group CONCLUSIONS: Preliminary findings suggest that the beneficial effect of an unguided brief MEMI to target pathological worry, trait mindfulness, and EF is modest yet potentially meaningful. Other theoretical and clinical implications were discussed.