Jefferson County is located in the eastern portion of the state of Missouri and is part of the St. Louis, MO-IL Metropolitan Statistical Area and encompasses many of the city’s southern suburbs.
From United States Census Bureau Quick Facts, Jefferson County, Missouri http://www.census.gov/quickfacts/table/PST045215/29099,12065,00
The population of Jefferson County as per the April 1, 2010 Census was 218,733, making it the 6th most populous county in Missouri. The county seat of Jefferson County is Hillsboro, MO.
The population estimate of Jefferson County as of July 1, 2014, was 222,716 for a change/growth of 1.8%.
Cool fact—Jefferson County was once the mean center of the U.S. population!: