Mind wandering (MW)—spontaneous, self-generated thought—is phenomenologically effortless, yet paradoxically linked to cognitive control, a process typically associated with mental effort. Influential accounts posit that cognitive control sustains MW by shielding internal thoughts from external interference and organizing them into a connected sequence. However, this raises a puzzle: if cognitive control is effortful, why does MW feel effortless? I resolve this tension by integrating the segmented structure of MW with the opportunity cost theory of mental effort. While cognitive control incurs mental effort, which reflects the opportunity cost (the value of forgone alternatives), MW’s segmented structure—thematically clustered thoughts forming brief segments, separated by topic shifts—prevents sustained investment of cognitive resources in any single segment/topic. Each segment briefly engages control, but rapid transitions reset costs before they accumulate to the extent that leads to phenomenological effortfulness. Thus, MW leverages control transiently across discrete segments, reconciling its reliance on control with its low-effort quality. This reframes the cognitive control view: control operates locally within each segment, not globally across the entire MW episode. My account thus deepens our understanding of MW and resolves the tension between the typical effortfulness of cognitive control and MW’s phenomenological effortlessness.