Objective: To assess whether the French "Amis aussi la nuit" ("Friends also at night") campaign on peer-support concerning binge drinking and cannabis use in a party setting reached young adults aged 17–25 years, and how it was perceived by them. Methods: A posttest evaluation of the campaign using a mixed-methods design was performed. Quantitative data from a large survey (n = 1003) and qualitative data from semi-structured interviews (n = 23) and one focus group (n = 6) were combined to explore strengths and weaknesses of the campaign. We adopted a convergent design where all data were collected simultaneously, reported jointly and, finally, cross-checked for discussion. Results: Participants appreciated the campaign, felt involved in its preventive messages and found used communication strategy acceptable and effective. Some negative features were also identified, especially concerning the language and tone of the campaign. Conclusions: Recommendations for improving this campaign and informing future ones on similar topics are provided based on the study results. Our findings are important for public health professionals who are interested in the prevention of high-risk behaviors in the young. © 2022 Taylor & Francis Group, LLC.