College students are at a high risk of mental health problems due to continuous exposure to considerable stress as they transition into adulthood. It is necessary to reflect on young people’s needs and provide brief, personalized support interventions via mobile applications. This study aimed to (1) describe the co-design development process of a behavioral activation (BA) mobile health application called MEndorphins to help youth manage stress; and (2) evaluate the ease of use and quality of the application and its effects on psychological distress. College students aged 18–25 in South Korea participated as co-designers throughout the MEndorphins development process, which involved prototyping workshops. Thirty-five students also evaluated the application’s ease of use and quality, as well as its effects on psychological distress, using a self-reported online questionnaire. In the pilot evaluation, ease of use scored 74.21 out of 100 and quality 3.72 out of 5. There were statistically significant decreases in depression, anxiety, and stress after using MEndorphins (p ≤ 0.001 for depression and anxiety, p = 0.001 for stress) for 7 days. In this developed BA based mobile application, participants could monitor their mood, plan stress self-management strategies, and gain motivation by sharing experiences.