Autism spectrum disorder (ASD) is characterized by impairments in social affective engagement. The present study uses a mild social stressor task to add to inconclusive past literature concerning differences in affective expressivity between autistic young adults and non-autistic individuals from the general population (GP). Young adults (mean age = 21.5) diagnosed with ASD (n = 18) and a non-autistic comparison group (n = 17) participated in the novel social stress task. Valence (positive/negative) and intensity of facial affect were coded across four observational episodes that alternated between engagement and disengagement of social conversational partner. Results indicated an overall attenuation in expressivity in the ASD group in comparison to the non-autistic group. Mean affect differed between groups, especially in the amount of affective expression. Both groups responded with increased positive expressions during social engagement episodes. The affect difference was driven by a smaller proportion of positive and a greater proportion of neutral affect displays in the ASD group compared to the non-autistic group during these episodes, and less so by negative affect differences. The results suggest that friendly, non-threatening social interactions should not be assumed to be aversive to autistic individuals, and that these individuals may respond to such situations with muted positive valence. These findings are consistent with past reports of decreased expressivity in autistic individuals compared to individuals from the general population, specifically in an ecologically valid social context.