Mount Carroll is a city in Carroll County, Illinois, United States. It is the Carroll County seat. The population was 1,717 at the 2010 census and as of 2018, the population had dropped to an estimated 1,587.Due to its elevation and northwesterly location, Mount Carroll is subject to unusually cold winter weather. From 1930 to 1999, Mount Carroll held the record for the lowest temperature ever recorded in Illinois, −35 °F (−37 °C), recorded on January 22, 1930. The record was beaten by Congerville in 1999, by one degree Fahrenheit. 20 years later, on January 31, 2019 during an extreme cold snap, Mount Carroll regained the title of coldest city in Illinois when a new Illinois state record low temperature of −38 °F (−39 °C) was officially recorded.