A number of studies have demonstrated the mental health implications of excessive Internet-browsing, gaming, texting, emailing, social networking, and phone calling. However, no study to date has investigated the impact of being able to conduct all of these activities on one device. A smart-device (i.e., smart-phone or tablet) allows these activities to be conducted anytime and anywhere, with unknown mental health repercussions. This study investigated the association between smart-device use, smart-device involvement and mental health. Two-hundred and seventy-four participants completed an online survey comprising demographic questions, questions concerning smart-device use, the Mobile Phone Involvement Questionnaire, the Internet Addiction Test and the Depression, Anxiety and Stress Scales. Higher smart-device involvement was significantly associated with higher levels of depression and stress but not anxiety. However, smart-device use was not significantly associated with depression, anxiety or stress. These findings suggest that it is the nature of the relationship a person has with their smart-device that is predictive of depression and stress, rather than the extent of use.