Previous studies have suggested that the dorsomedial prefrontal cortex (dmPFC) plays a central role in processing first impressions; however, little is known about how dmPFC processes different valences of first impressions. Moreover, it is still unclear as to whether the dmPFC shows lateralization or only induces different levels of activation when processing positive and negative impressions. To address these questions in the present study, the brain activities for the impression judgments expressed by participants were measured with near-infrared spectroscopy. For each real facial picture, participants were asked to evaluate their first impressions on a scale from ‘bad’ to ‘good’ using a keyboard. The results showed that although the right dmPFC has a higher sensitivity in processing impressions, both the hemispheres of dmPFC showed a significant trend where the activation of positive impressions was higher than the negative ones. Accordingly, it is proposed that the dmPFC acts as a single mechanism responsible for delineating the processing of first impressions rather than two lateralized systems. Therefore, a ‘positivity dominance hypothesis’ is also proposed, which states that dmPFC in both hemispheres have a higher sensitivity and priority for positive impressions than negative ones. The present study provides valuable findings with respect to the role of the dmPFC in the processes of first impression formation.