In 2019, the most populous county in Missouri is St. Louis County with a population of 996,945. This represents modest population decline of 0.19% since the 2010 census. Other major counties in Missouri include Jackson County (700,307), St. Charles County (399,182), and St. Louis City (302,838). These are the only counties with more than 300,000 residents, although there are eight other counties with at least 100,000 residents. Of these counties, St. Charles County experienced the most rapid growth at 10.33% since 2010.
The smallest Missouri county in terms of population is Worth County with just 2,040 residents. This represents a growth rate of -5.20% since 2010. Missouri has six other counties under 5,000 residents including Mercer County (3,641), Knox County (3,947), and Holt County (4,404). All of these counties lost residents since 2010 with the most dramatic decline in Holt County with a growth rate of -10.40%.
The fastest-growing county in Missouri since the 2010 census is Platte County with a growth rate of 14.81%, bringing its total population to 102,985. Other counties with rapid growth include Clay County grew 10.63% to 246,365, and St. Charles County grew 10.33% to 399,182, and Boone County grew 10.30% to 180,005. The county with the most significant decline was Pemiscot County which lost 10.94% of its residents.