This study evaluated the effectiveness of a Mahayana Buddhist teaching- and group-based intervention, the Awareness Training Program (ATP), which is textually aligned to a Mahayana Sūtra so that its theory and practice are coherent. The ATP aims to alleviate stress by enhancing participant’s compassion and wisdom of nonattachment. Middle-aged working adults (n = 122) in Hong Kong participated in this randomized waiting-list controlled trial. Self-reported psychological questionnaires were used to assess the participants’ level of stress (PSS), sense of coherence (SOC), psychological well-being (GHQ), and nonattachment (NAS) at pretest, posttest, and 3 months later. The data showed significant improvements in the intervention group over the controls for all outcome measures at posttest and 3 months later. A mediation analysis demonstrated that nonattachment mediated both the treatment and the maintenance effects for all outcome variables.