When Louisiana was admitted as the 18th state in 1812, the vast territory acquired from France in 1803 became the Missouri Territory with five counties and St. Louis as its capital. In 1813, Washington, and in 1816, Howard Counties were added to the five original counties: Cape Girardeau, New Madrid, St. Charles (including what is now Montgomery), St. Genevieve and St. Louis.
The 1818-1819 Territorial Legislature petitioned congress for authority to form a state government. Eight additional counties were organized: Cooper, Franklin, Jefferson, Lincoln, Madison, Montgomery, Pike and Wayne, in a configuration close to the shape of present day Missouri.
Montgomery County was officially established 14 December 1818.
It included non-county Area 3, attached "for judicial, probate, administration and county purposes" (Mo. Terr. Laws, 1818-19, Ch 230, Secs 1, 8, PP 580-84). The County and Administrative Area was the entire southwest corner of St. Charles County.