Background: The average age at menarche (AAM) among girls is influenced by various factors, including the socioeconomic status of the country, geographical location, and anthropometric characteristics. In Mongolia, limited research has been conducted on this topic Aim: This study aimed to compare the mean age at menarche among girls living in Bayanzürkh District of Ulaanbaatar and those in Uvs Province. Materials and Methods: A cross-sectional study was conducted between April and June 2025. The study participants included schoolgirls aged 8 years and older from general education schools located in Bayanzürkh District and Uvs Province. Statistical analyses were performed using R version 4.5.1 (RStudio 2025.05.1-513) Results: A total of 486 girls participated in the study. The mean age at menarche among the participants was 12.23±1.29 years. The mean age at menarche among girls aged 8–18 years in Bayanzürkh District was 11.84±1.19 years, while that among girls aged 8–18 years in Uvs Province was 12.97±1.15 years. Thus, girls in Bayanzürkh District experienced menarche 1.13 years earlier on average than those in Uvs Province. The difference in mean AAM between urban (Ulaanbaatar) and rural (Uvs Province) girls was statistically significant (p<0.0001). Conclusion The mean age at menarche among girls residing in Bayanzürkh District was significantly lower than that of girls living in Uvs Province, indicating a tendency toward earlier onset of menarche in urban areas.