The objective of this study was to explore the impacts that clients experience after participating in experiential art therapy groups and to describe which processes affect these experiences. The research question was: How do clients describe the impacts of their participation in an art therapy group? The data was gathered in two phases. The first narrative material consisted of the retrospective writings of participants (n = 68) from 12 different art therapy groups. In the systematic qualitative content analysis of the narrative data, 12 main themes of impacts described by clients were identified. In the second round of data acquisition, the trustworthiness of the content analysis was tested by conducting a survey of the groups’ participants. A self-report questionnaire with structured and open-ended items was developed based on the main impact themes. The response rate for the questionnaire was 63.75% (n = 51). Of the respondents, 98% felt that art therapy had a positive impact on their psychological health, 82% recognised positive effects on their social relationships, and 67% noted such effects on their physical health. Finally, triangulation of the content analysis with the survey enabled the comparison and synthesis of the results into an overview of the impacts experienced and the processes mediating them.