Emotional expressions evoke rapid facial reactions in the perceiver that are consistent with the valence of the observed expression. We aimed to investigate whether this robust facial reaction is purely a motor matching response or instead represents underlying affective processes. Participants’ (N = 60) corrugator supercilii and zygomaticus major muscle activity was quantified using facial electromyography (EMG) while they viewed three sets of images; (i) upright happy and angry facial expressions, (ii) inverted happy and angry facial expressions, and (iii) sad and happy eyes and mouth expressions. Participants displayed patterns of EMG responding that were consistent with the affective valence of the emotional expression, as opposed to merely matching the observed stimuli (i.e. a motor matching response). Using a novel methodological approach, these findings provide evidence for the contention that affective processing underlies rapid facial mimicry reactions.