Mindfulness interventions have been shown to improve several subcomponents of attention; however, the psychological mechanisms driving these improvements are unknown. Mindfulness interventions train individuals to monitor present moment experiences while adopting an attitude of acceptance toward these experiences. We conducted a theoretically driven randomized controlled trial to test the putative mechanisms of mindfulness training that drive improvements in attentional control. Participants were randomly assigned to 1 of 3 conditions: (a) monitor and accept (MA) training, a standard 8-week mindfulness-based stress reduction (MBSR) intervention that included cultivation of both monitoring and acceptance skills; (b) monitor only (MO) training, a well-matched modified 8-week MBSR-adapted intervention that focused on monitoring skills only; or (c) no treatment (NT) control. Momentary attentional control was measured via ecological momentary assessment for 3 days at baseline and postintervention. Trait attentional control was assessed at baseline and postintervention using traditional self-report. Participants also completed a dichotic listening task to assess sustained attention at baseline and postintervention. We found that MA and MO participants improved in momentary and trait attentional control (but not attention task performance) relative to NT participants. Analyses of indirect effects were consistent with the possibility that increased momentary attentional control partially accounts for MA/MO intervention-related increases in trait attentional control. This randomized controlled trial provides one of the first experimental tests of the mechanisms of mindfulness interventions that drive improvements in attention outcomes. These findings support the notion that present-focused monitoring skills training drives mindfulness intervention-related improvements in momentary attentional control, which in turn fosters greater trait attentional control. (PsycInfo Database Record (c) 2021 APA, all rights reserved).