Mental health is an increasingly prevalent topic of public interest, but remains a complex area requiring focused research that must account for negative perceptions surrounding mental health issues. The current work explores the roles of social media information source credibility and valence of social media comments on health outcomes in such a mental health context. We used a 2 (message source: professional vs. layperson) x 3 (valence of comments: positive vs. negative vs. mixed) online experiment to examine the effects of source and valence of comments on trust, attitudes and intentions related to mental health information and services among 422 undergraduate students. Results supported the hypothesized model in which source influenced cognitive trust while comments influenced affective trust. Cognitive and affective trust both impacted attitudes towards mental health information which encourages the intention to share such information on social media. Additionally, affective trust impacted attitudes towards mental services which influenced intentions to seek them out. Source and valence of comments on social media impact different behavioral intentions regarding the use of mental health services. This study provides insights for future social media campaigns promoting mental health service use.