Mental health problems like anxiety, depression, and stress have been increasing in many countries and the 2020 COVID-19 pandemic has further exacerbated their toll. Mindfulness-based interventions have been shown to provide evidence-based treatments for anxiety and depression, and accumulating evidence is emerging in support of using mindfulness apps yielding small-to-moderate treatment effects. The study was a 4-week randomized controlled trial with 561 university students and staff as participants, divided into a treatment group (mindfulness app) and an active control group (psychoeducational online content). Depression, anxiety, and stress were evaluated as primary study outcomes. Saliva cortisol samples were also collected from a subgroup of the treatment arm (n = 29). Using the mindfulness app for four weeks resulted in small reductions in stress (d = .16), and depression (d = .16). Attrition was 28.0%. Subjects who practiced more did not experience additional improvement in wellbeing. Mindfulness apps offer modest but clear benefits to users in terms of improved mental health. They present a promising supplement to traditional mental health services.