Adolescents spend an increasing amount of time on mobile phones. The aim of this study was to assess the relationship between duration of mobile phone use and insomnia and depression in senior high school students. The cross-sectional study was conducted on 295 senior high school students in Japan. Mobile phones were owned by 98.6% of students; 58.6% of students used mobile phones for more than 2 h daily and 10.5% used them for 5 h daily. The risk of insomnia was significantly high in students who used mobile phones for 5 h or more (OR: 3.89 [95% CI: 1.21-12.49]). There was no significant association between depression and duration of mobile phone use. However, individuals who spent 120 min or more using mobile phones for social network services (OR: 3.63 [1.20-10.98]) and online chats (OR: 3.14 [1.42-6.95]) were at risk for depression. Excessive mobile phone use is associated with unhealthy sleep habits and insomnia. Moreover, the excessive use of the social network services and online chats are related to depression rather than duration of mobile phone use. Adolescents should use mobile phones appropriately to avoid sleep disturbances and the impairment of mental health.