Smartphone applications may ensure greater access to services for the many people who experience anxiety and other psychological issues but do not receive adequate treatment. ACTCompanion® is a mHealth application that incorporates Acceptance and Commitment Therapy principles into daily exercises that assist the user to confront their negative internal experiences and work towards valued outcomes. We used a single-subject A-B design with nine students and 10 employees experiencing high daily anxiety. Participants completed a battery of questionnaires before and after app use. They completed the DAS-A regularly so we could monitor anxiety levels over time. Daily anxiety levels reduced in the intervention phase, and remained low for six participants for whom follow-up data were available. There was also a significant improvement in scores of anxiety, depression, stress, positive and negative effect, psychological flexibility, and mindfulness. Our results suggest that mHealth applications have potential to improve people’s psychological functioning.