Mind wandering—the drift of thought away from an ongoing task or while at rest—is often regarded as a mere lapse in cognitive control. This perspective is challenged by reframing mind wandering as a subtle yet significant expression of spontaneous agency. A detailed structural analysis reveals that a mind wandering episode typically consists of several segments, each involving distinct topics (semantic content) and diverse mental activities (e.g., memory, imagination, planning), often connected by automatic associations. This structural view allows for a precise determination of mind wandering’s controlled and spontaneous components. While the initiation of the entire episode and the transitions between segments are normally automatic, the process is not devoid of control. The whole episode occurs under the condition of perceptual decoupling, which relies on inhibitory control, and the unfolding of each segment is supported by working-memory capacity. Critically, the content of these segments is far from random; instead, it is often guided by personal goals and concerns. Therefore, mind wandering can be understood as an emergent mental process, arising from the specific dynamic interaction between controlled elements (such as goal representations, working memory, and inhibitory control) and spontaneous elements, thereby constituting spontaneous agency.