Clinical perfectionism is associated with various cognitive processes including performance monitoring and emotion regulation. This exploratory study analyzed neurological data from a randomized controlled trial for clinical perfectionism that compared acceptance and commitment therapy (ACT) to a waitlist control. The objective was to assess the effect of ACT on neural activation. Twenty-nine participants underwent a functional near-infrared spectroscopy assessment during which they completed behavioral tasks designed to elicit error detection and error generation at pre- and posttreatment. The hemodynamic response function (HRF) in the dorsolateral prefrontal cortex, dorsomedial prefrontal cortex, and right inferior parietal lobe was analyzed using mixed effects models. In all areas, we found reductions or smaller increases in the total HRF for experimental tasks from pre- to posttreatment in the ACT condition compared to the waitlist condition. Decreases in total oxygenated hemoglobin are consistent with diminished recruitment of neurons in response to previously emotionally salient stimuli, possibly representing greater cognitive processing efficiency. Our preliminary findings tentatively support the processes of change posited by the theory underlying ACT and highlight the need for more precise methodology in neurological assessment to adequately evaluate how treatment affects neurological function. Limitations include lack of an active comparison condition and behavioral data.