The stress of transitioning to college can worsen mental health. We examined whether a brief mobile-app mindfulness intervention combined with mood monitoring was more effective in alleviating first-year students’ psychological distress than mood monitoring alone. Participants (#NCT06348277, 3/28/24 retrospectively registered) were 88 first-year students (Mage = 18.51 years; SD = 0.64; 60.5% White; 65.9% women) randomly assigned to a mindfulness-plus-mood-monitoring or a mood-monitoring-only group and asked to use an app 3 times a day for 3 weeks. They completed self-report questionnaires at baseline, post-intervention, and 6 and 12 weeks later. Students in both conditions had significant reductions in symptoms of depression and anxiety, urges to self-injure, and rumination. These effects lasted throughout the 12-week follow-up, mostly with medium to large effect sizes. There was no change in worry in either intervention group. It appears that mood monitoring, with and without brief mindfulness exercises, is effective in reducing negative affect, rumination, and urges to self-injure among first-year college students.