The Missouri Constitution of 1820 was written in 38 days in the City of St. Louis, creating three branches of state government. In the document, portions of Article III, Section 26 proved problematic in the eyes of the U.S. Congress. However, the concern that Congress had regarding the section would be resolved and Missouri was officially admitted into the Union on August 10, 1821.
Based on the limited research conducted, it appears that the 1820 Constitution was never amended during its' tenure as the governing instrument of Missouri until it was replaced 1865. With this said, new research has determined that in Jefferson City, between November 1845 and January 1846 a constitutional convention occurred attempting to address the issue of the limits on the creation of new counties in the-then current constitution. This proposed constitution was voted upon by the citizens of the state in August 1846 but failed to be ratified.
The text of the 1820 Constitution is available here in four parts. Part One includes the Preamble and Articles I (1) through III (3). Part Two contains Articles IV (4) through VIII (8). Part Three contains Articles IX (9) through XIII (13) while Part Four contains a schedule (of events) and the “signatures” of the 1820 Constitutional Convention delegates.