Horticultural therapy (HT) has long been used in the rehabilitation of people with mental illness, but many HT programs are not standardized, and there have been few evaluation studies.This study evaluated the process and outcomes of a standardized horticultural program using a mixed methodology, i.e., systematic integration ("mixing") of quantitative and qualitative data within a study.Participants who have mental illnesses were assigned to a treatment (HT) and a comparison group ( = 41 for each group). The process and outcomes of the program, including stress and anxiety, engagement and participation, affect changes, mental well-being, and social exchange, were obtained using self-completed questionnaires, observational ratings of participants during the group, as well as through a focus group.The study results supported the proposal HT is effective in increasing mental well-being, engagement, and the sense of meaningfulness and accomplishment of participants. Many participants reported a reduction in stress and anxiety in the focus group, but positive changes in affect were not fully observed during the group process or captured by quantitative measures. The participants also did not report increases in the social exchange over the HT sessions.The evidence supports that HT is effective in increasing mental well-being, engagement in meaningful activities, but did not result in significant affect changes during therapy, or increase social exchanges among people with mental illness.