Byrnes Mill is a city in Jefferson County, Missouri, United States. The population was 2,781 at the 2010 census.
Byrnes Mill is located at 38°26′7″N 90°34′26″W (38.435197, -90.573838).
According to the United States Census Bureau, the city has a total area of 5.26 square miles (13.62 km2), of which 5.14 square miles (13.31 km2) is land and 0.12 square miles (0.31 km2) is water.
The city is located on the Big River, and has the ruins of an old mill and dam.
As of the census of 2010, there were 2,781 people, 1,038 households, and 782 families living in the city. The population density was 541.1 inhabitants per square mile (208.9/km2). There were 1,121 housing units at an average density of 218.1 per square mile (84.2/km2). The racial makeup of the city was 97.4% White, 0.3% African American, 0.5% Native American, 0.3% Asian, 0.5% from other races, and 1.0% from two or more races. Hispanic or Latino of any race were 1.4% of the population.
There were 1,038 households, of which 34.7% had children under the age of 18 living with them, 60.0% were married couples living together, 9.7% had a female householder with no husband present, 5.6% had a male householder with no wife present, and 24.7% were non-families. 19.6% of all households were made up of individuals, and 4.9% had someone living alone who was 65 years of age or older.