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Description: Research on gaze control has long shown that increasingly difficult visual-cognitive processing in scene viewing is associated with longer fixation durations. More recently, though, longer fixation durations have also been linked to mind wandering, a perceptually decoupled state of attention marked by attenuated visual-cognitive processing. Toward resolving this apparent conflict, we ran simulations using an established random-walk model for saccade timing and programming and assessed which model parameters best predicted modulations in fixation durations associated with mind wandering compared to attentive viewing. We showed that mind wandering-related fixation durations were best described as an increase in the variability of the fixation-generating process, leading to more variable—sometimes very long—fixation durations. These findings illustrate how mind wandering impacts the stability of fixation durations, highlight the importance of variability measures toward understanding gaze control, and suggest that longer fixation durations from mind wandering versus processing difficulties emerge from distinct mechanisms.