The Great Qing Imperial Post Office was established in 1896 and it started operating the following year. After the Boxer Uprising, the Post Office went through two large expansions. Outer Mongolia was not included in either expansion until a dramatic change occurred in the first year of Xuantong Emperor (1910). Through reference to a combination of historical documents and postal artefacts, this article explores the reasons behind the establishment of the first postal route between Kulun (Urga, today’s Ulaanbaatar) and Zhangjiakou (Kalgan), and the story of its first day of operation. In so doing, it seeks to demonstrate that the Qing Government used the extension of the postal service to Outer Mongolia to assert its sovereignty over Mongolia.