Mind wandering is a pervasive feature of human cognition often associated with the withdrawal of task-related executive control processes. Here, we explore the possibility that, in tasks requiring executive control to sustain consistent responding, moments of mind wandering could be associated with moments of increased behavioral variability. To test this possibility, we developed and administered a novel task (the metronome response task) in which participants were instructed to respond synchronously (via button presses) with the continuous rhythmic presentation of tones. We provide evidence (replicated across 2 independent samples) that response variability during the 5 trials preceding probe-caught reports of mind wandering (tuned-out and zoned-out mind wandering) is significantly greater than during the 5 trials preceding reports of on-task performance. These results suggest that, at least in some tasks, behavioral variability is an online marker of mind wandering. (Contains 2 figures and 2 footnotes.)