Autistic adults commonly experience mental health concerns including social anxiety and depression, which can have negative effects on their quality of life. It is not completely clear, however, why rates of mental health concerns are so high. Some evidence suggests that social connectedness might play a key role. The goal of this study was to explore links between loneliness, mental health concerns, autism features, and social contact among autistic adults and test whether the links between mental health with autism features and social contact can be explained by loneliness. Researchers in this study collected data using questionnaires completed by 69 autistic young adults. Autistic adults who reported more autism features also reported more social and family loneliness, higher levels of social anxiety and depression, and fewer initiated social contacts. In addition, adults with more social contact initiations were likely to report lower levels of social and family loneliness and social anxiety but not depression. Results showed that the link from social engagement and autism features to social anxiety and depression symptoms could be mostly explained by loneliness. The results of this study expand previous findings by illustrating one factor (loneliness) that might be responsible for the high rates of mental health concerns among adults on the autism spectrum. These findings highlight the importance of studying factors related to mental health concerns among autistic adults and ways to best support social connectedness for the mental well-being of autistic young adults.