Warren County is a county in the U.S. state of Illinois. According to the 2020 census, it had a population of 16,835. Its county seat is Monmouth. Warren County was organized in 1825 out of Pike County which consisted of all portions of the state north and west of the Illinois River before 1825. Henderson County was formed in 1841 from the western area of War…Warren County is a county in the U.S. state of Illinois. According to the 2020 census, it had a population of 16,835. Its county seat is Monmouth. Warren County was organized in 1825 out of Pike County which consisted of all portions of the state north and west of the Illinois River before 1825. Henderson County was formed in 1841 from the western area of Warren County.
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