Objectives This study examined the influence of adult attention-deficit/hyperactivity disorder (ADHD) and insomnia on the risk of unintentional minor musculoskeletal injuries (UMMI). Methods A prospective observational study was carried out in a single unit of the army in South Korea. One hundred and seventy-seven subjects participated in the study. The Adult ADHD Self- Report Scale (ASRS-v1.1) and Insomnia Severity Index (ISI) was used to screen. The risk of UMMI during one-week winter ranger training was investigated. Results All subjects were male and aged 21.75 (standard deviation 1.5). Approximately 9.6% of subjects had clinically relevant adult ADHD symptoms. Subjects with ADHD had a higher stress index and higher rates of insomnia than those without symptoms. After adjusting for age, rank, service months, obesity, history of UMMI, sleep time, and risk of the task, the ADHD group was 4.90 times more likely to have UMMI than the control group. Insomnia increased the risk of UMMI 8.14 fold. Conclusion These results suggest that an evaluation and intervention for adult ADHD and insomnia should be made in people engaged in other tasks that may result in UMMI.