Choosing between equivalent response options requires the resolution of ambiguity. One could facilitate such decisions by monitoring previous actions and implementing transient or arbitrary rules to differentiate response options. This would reduce the entropy of chosen actions. We examined voluntary action decisions during magnetoencephalography, identifying the spatiotemporal correlates of stimulus- and choice-entropy. Negative correlations between frontotemporal activity and entropy of past trials were observed after participants' responses, reflecting sequential monitoring of recent events. In contrast, choice entropy correlated negatively with prefrontal activity, before and after participants' response, consistent with transient activation of latent response-sets ahead of a decision and updating the monitor of recent decisions after responding. Individual differences in current choices were related to the strength of the prefrontal signals that reflect monitoring of the statistical regularities in previous events. Together, these results explain individual expressions of voluntary action, through differential engagement of prefrontal areas to guide sequential decisions.