Spontaneous mimicry, including that of emotional facial expressions, is important for socio-emotional skills such as empathy and communication. Those skills are often impacted in autism spectrum disorders (ASD). Successful mimicry requires not only the activation of the response, but also its appropriate speed. Yet, previous studies examined ASD differences in only response magnitude. The current study investigated timing and magnitude of spontaneous and voluntary mimicry in ASD children and matched controls using facial electromyography (EMG). First, participants viewed and recognized happy, sad, fear, anger, disgust and neutral expressions presented at different durations. Later, participants voluntarily mimicked the expressions. There were no group differences on emotion recognition and amplitude of expression-appropriate EMG activity. However, ASD participants' spontaneous, but not voluntary, mimicry activity was delayed by about 160 ms. This delay occurred across different expressions and presentation durations. We relate these findings to the literature on mirroring and temporal dynamics of social interaction.