Mason County is a county in the U.S. state of Illinois. According to the 2020 census, it had a population of 13,086. Its county seat is Havana. The county is named in honor of George Mason, a member of the Virginia legislature who campaigned for the adoption of the United States Bill of Rights.

Havana is a city and the county seat of Mason County, Illinois, United States. The population was 2,963 at the 2020 census, a decline from the figure of 3,301 tabulated in 2010.