In Mongolia, there is limited data on the prevalence and correlates of common mental health conditions. This study addresses this data gap by exploring anxiety, depression, and brain overwork. The aim of this study was to determine normative data on these conditions in the general population of Mongolia. This nationwide, population-based, cross-sectional study was conducted in 48 sampling centers across Mongolia in 2020. A total of 613 participants (190 men and 423 women) with a mean age of 41.8 ± 12.4 years were recruited. The participants completed the Hospital Anxiety and Depression Scale (HADS) and the Brain Overwork Scale (BOS-10). Vital signs, body measurements, and lifestyle determinants were also assessed. The prevalence of anxiety was 9.9%, depression was 4.9%, and brain overwork was 18.3% among the participants. Anxiety and depression were correlated with brain overwork symptoms. Brain overwork was associated with young age, unemployment, low income, and alcohol use. These findings suggest that anxiety, depression, and brain overwork are a significant problem in the general population of Mongolia. Further research is needed to develop effective interventions to reduce the prevalence and risk factors of anxiety, depression, and brain overwork.